OH density, flux and loss probability measurements in a room temperature atmospheric pressure surface discharge by microscopic laser induced fluorescence

Many applications involving atmospheric pressure plasma-substrate interactions are enabled by the large fluxes of short-lived reactive species such as OH radicals to the substrate, nonetheless, the accurate measurement of radical densities and fluxes at substrates at atmospheric pressure has received little attention to date, particularly for surface ionization waves. We report the measurement of the OH density distribution in a surface discharge on a fused silica (quartz) substrate generated by an impinging atmospheric pressure plasma jet in dry and humid helium. The OH density is measured by microscopic laser induced fluorescence with a spatial resolution of 10 µm in the direction perpendicular to the quartz substrate. The measured OH diffusive flux varied for the investigated experimental conditions by almost three orders of magnitude and had a maximum value of 1.7 × 1015 cm−2 s−1. The corresponding surface loss probability of OH on the quartz surface was determined to be ∼0.01. The high spatial resolution was required to accurately resolve the near surface gradient of OH radicals.


Introduction
Low temperature atmospheric pressure plasma jets (APPJ) are an abundant source of reactive species enabling numerous applications including environmental remediation [1][2][3], * Author to whom any correspondence should be addressed.
Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. surface treatment [4,5] and decontamination applications [6,7]. This wide range of applications is due to the unique ability of APPJs to interact with a broad range of substrates. Dielectric barrier and corona discharges have been extensively used for surface treatments [8,9]. In this case, the plasma is generated between two electrodes of which one is the substrate. However, APPJs allow for the remote generation of plasma in an open air surrounding through sustaining guided streamers or plasma effluents in a noble gas channel [10]. When the plasma is not in direct contact with the substrate, the interaction of plasma jets with substrates can be seen as convective transport of reactive species through the gas jet impinging on the substrate [11]. Nonetheless, when the plasma is in direct contact with the substrate, a more complex interaction of plasma with the substrate occurs often for dielectric substrates leading to the formation of surface ionization waves and surface streamers [12,13].
Understanding the mechanisms of plasma-surface interactions requires a detailed knowledge of the reactive species flux to the substrate. Both the boundary layer in the impinging jet zone in which short-lived species are often depleted compared to the bulk plasma and transport is dominated by diffusion rather than convection [14] and the thin plasma sheath structures are typically on the order of 100 µm or smaller [11,15]. Hence, the measurement of the reactive species fluxes to a substrate at atmospheric pressure requires a high spatial resolution. In addition, the species flux at the interface can be significantly impacted by recombination reactions in the boundary layer and surface reactions/losses [16]. As surface reactions often strongly depend on surface properties and are less studied than gas phase reactions, it is critical that reactive species fluxes derived from models are compared with experiments.
Most studies in the field of plasma science use the concept of surface loss probability, β to describe surface reactions [16][17][18][19]. As the surface loss of radicals can dominate volume loss processes in low pressure plasmas, the surface loss probability has been studied extensively in low pressure [19,20]. It has been reported that surface loss probabilities are impacted by the surface properties [19][20][21], gas pressure and composition [20,22,23] and temperature [24,25]. For example, Guerra used an analytic model to describe the surface recombination of N and O for a broad range of conditions from low to atmospheric pressure [26]. The reported results underline the complex dependence of surface recombination coefficients with temperature and pressure due to the transition from Eley-Rideal to Langmuir-Hinshelwood recombination mechanisms or vice-versa.
The measurement of surface loss probabilities or surface recombination coefficients have been achieved through catalytic probe measurements [27], molecular beam measurements complemented with molecular fluorescence imaging [28] or low-pressure plasma experiments (for conditions where surface losses dominate) with gas phase radical species density detection [29]. High-resolution laser induced fluorescence (LIF) is one of the key methods to measure the spatial gradients in radical species densities near substrates. This approach typically uses a laser beam or sheet which allows imaging the radical distribution in the region close to the substrate [30][31][32]. While surface reactions have been extensively studied in plasma-catalysis and catalytic combustion at high pressure [33], surface loss probabilities in atmospheric pressure plasmas have received little attention. This is likely due to the more challenging measurement conditions compared to low pressure. A notable exception is the work of the Suzuki group, they measured the OH loss coefficient through near substrate LIF measurements for a quartz substrate in high temperature combustion and plasma [34][35][36]. As the surface site density and loss coefficient can have a strong function of temperature, it is unclear whether the derived surface loss coefficients of OH are also valid for room temperature atmospheric pressure plasmas where OH radicals often play a key role [1,5,17].
In this paper, we report microscopic LIF of OH radicals in an atmospheric pressure surface ionization wave on a quartz substrate. The obtained spatially and temporally resolved OH density measurements not only allow us to investigate the OH generation in a surface ionization wave discharge but also obtain the OH flux to the substrate. Furthermore, the measured fluxes and densities enable us to determine the OH surface loss probability for room temperature atmospheric pressure plasmas for the first time, to our knowledge. A key difference in the OH LIF implementation in this study compared to previous work is that the laser beam is perpendicular to the quartz surface to allow an improved spatial resolution and sensitivity at the substrate. Figure 1 shows a schematic of the plasma setup, substrate, and diagnostics used in this work. A quartz plate with dimensions 76.4 mm × 25.5 mm × 1 mm was used as the substrate. The quartz substrate was investigated under plasmasurface interaction conditions open to the ambient air and has not been characterized in detail. For example, the quartz substrate might have absorbed water molecules which have not been considered in the analysis in this work. Nonetheless, we ensured steady state conditions were reached during the measurements as confirmed by highly reproducible measurements during long duration experiments. The atmospheric pressure plasma was generated by a plasma jet in a quartz capillary with an inner diameter of 1.8 mm. The configuration of the jet was similar to the conditions reported in [31,37]. The high voltage electrode was a needle electrode with an inner diameter of 1 mm and the tip of the needle was at 4.4 mm from the nozzle of the quartz capillary. A grounded ring electrode with a length of 5 mm was placed around the quartz tube with the tip of the high voltage needle in the center of the ring electrode. The plasma jet was positioned at an angle of 45˚from the quartz plate and the nozzle of the quartz tube was in contact with the quartz substrate. As shown in figure 1, the configuration of the setup enables the generation of a downward (in the direction of the gravitational force) ionization wave along the quartz substrate. The plasma jet was mounted on an x-yz translational stage to allow for the accurate positioning of the jet compared to the fixed substrate and the laser beam (see further). A helium flow was supplied through the quartz capillary at a flow rate of 0.725 standard liters per minute (slm). The gas supply system consisted of two mass flow controllers (MKS GE50A, maximum range: 0.5 and 2 slm) that allowed to flow helium (purity of 99.9995%) through the plasma jet or bypass a percentage of the helium through a washing bottle filled with distilled water to humidify the total helium gas flow with 0.28 ± 0.07% H 2 O. The mole percentage of water was obtained by assuming that the amount of water in the bypass helium gas flow was saturated with water vapor at room temperature.

Plasma setup
The high voltage pulses were generated by a high voltage pulse generator (PVX-4110) with an amplitude of 3 kV and a duration of 1 µs. The high voltage was provided by a DC high voltage power supply (Spellman SL300). The high voltage pulse generator was triggered by a signal generator (BNC Model 577). The signal generator is also used to synchronize the plasma generation with the optical diagnostics. The jitter of the time synchronization is about 1 ns. To allow time resolved measurements in the afterglow up to 5 ms, a burst of 95 pulses with a frequency of 1 kHz was applied and a 'dead time' between each burst cycle of 5 ms was introduced (10 Hz burst frequency). In this study, all the radical densities were measured 10 µs after the third voltage pulse in the burst except for the time resolved OH radical density measurements in the afterglow. The current and voltage waveforms were monitored with a Rogowski coil and in house made d-dot, respectively.

Optical diagnostics
LIF was used to measure the near interfacial spatial distribution of the OH and water vapor densities. The OH LIF approach and the calibration procedure as adopted in this work have been reported in [30,31]. A UV laser beam at ∼282 nm was used to excite OH (X, ν ′′ = 0) → OH (A, ν ′ = 1) which was generated by a frequency doubled laser beam at ∼564 nm produced in a dye laser (Sirah PrecisionScan) using Rhodamine 6 G dye and a second harmonic generation crystal. The dye laser was pumped by a Nd:YAG laser (QuantaRay Lab-170-10 H, full width half maximum (FWHM): 8 ns) at 532 nm. The UV laser beam was guided through the quartz slide using a spherical lens with a focal length of 300 mm resulting in a focus point at the position of the surface ionization wave. A knife edge was positioned to reduce stray light reflection on the plasma jet electrodes and capillary (see figure 1). The laser beam had an FWHM of 250 µm. All LIF measurements were performed with a laser energy of 5.8 µJ per pulse which was near the upper limit of the linear LIF regime for the investigated conditions.
The OH-LIF signals were recorded by an ICCD camera (Andor iStar 340) through a lens (Nikon 105 mm F4.5 UV). We inserted an extension tube with a length of 132 mm between the ICCD camera and the lens to achieve a resolution of 10 µm pixel −1 . A band pass filter (300 ± 10 nm, 15% transmission) was introduced between the plasma and the lens to reduce the background, laser scatter, and plasma emission contribution to the recorded fluorescence signal. The camera gate was set to 700 ns to collect the total OH fluorescence for absolute OH measurements and to 10 ns to measure the radiative lifetime of OH(A). Each collected OH fluorescence image consisted of an accumulation of 5940 laser pulses (dry helium) and 9900 laser pulses (humid helium). Imaging of the plasma emission was achieved with the same detection system using a camera gate of 5 ns although without filter enabling to collect emission between 220 to 850 nm. The gas temperature was measured by OH LIF using excitation LIF of the following transitions (Hund's case (a)): P 1 (1.5), P 1 (2.5), P 1 (3.5) and Q 1 (4.5) as described in detail in [30].

Implementation of delayed LIF measurement and calibration
A laser beam perpendicular to the substrate has many advantages for near substrate measurements including a high laser fluence light at the substrate position able to induce fluorescence. Nonetheless, this approach can lead to scattering from the substrate significantly impacting the LIF measurement near the substrate. To eliminate laser scattering from the LIF signal detection, the ICCD gate was delayed by 15 ns compared to the laser pulse. The advantage of this approach is illustrated in figure 2. Figure 2(a) shows a recording of a single shot laser pulse signal with the camera gate synchronized with the laser pulse (without bandpass filter). The single shot laser scattering intensity is of similar magnitude as the fluorescence intensity collected over more than 5000 pulses (figure 2(d)). The collected OH LIF signal with an ICDD gate delayed by 15 ns compared to the laser pulse (and detuned from the pumped transition), as shown in figure 2(b), illustrates that the scatter from the quartz tube can be removed. Figure 2(d) shows the OH LIF signal obtained by subtracting the plasma emission (figure 2(b)) from the measured LIF signal ( figure 2(c)). The resulting signal shows the ability, in spite of the delayed signal and significant quenching at atmospheric pressure, to record a LIF signal with a good signal-tonoise ratio without significant contributions of scattering and emission.
The absolute OH density was obtained through the calibration of the LIF signal by Rayleigh scattering in atmospheric pressure air at room temperature. To reduce the laser scattering from the substrate during the Rayleigh scattering measurement, the near substrate area (∼100 µm) was blocked with a knife edge. The Rayleigh intensity was assumed to be constant within 100 µm from the substrate. Vignetting was minimized by generating the plasma near the edge of the quartz substrate closest to the ICCD detector.
The impact of collisional transfer and LIF excitation dynamics was considered using a four-level LIF model [38]. As helium is an inefficient quencher of the laser excited OH(A) the quenching of OH(A), for the experimental conditions is dominated by ambient air (containing 1% humidity as measured in the temperature and humidity-controlled laboratory with a humidity meter) or water vapor in the case humid helium was used as feed gas through the jet. Figure 3(a) shows the OH(A) effective lifetime as a function of air concentration (including 1% humidity) in dry helium using the quenching coefficients reported in [38]. The effective lifetime in the range for air concentrations between 0.01 and 10% varies by two orders of magnitude and the reported relation between the OH(A) lifetime and air concentration allows to accurately determine the air concentration in the helium dry case. As we limit ourselves to conditions with a minimum lifetime of OH(A) of 7 ns (6% air with 0.28% water vapor), rotational energy transfer remains at least an order of magnitude faster than collisional quenching well within the validity range of the four level model used. In the humid helium case, we have two unknown gas concentrations (water vapor and air), and an assumption is needed to determine the gas composition. As in [31,39,40], we assume that at the same position, the dry and humid helium cases have the same air concentration. With this assumption, the changing water concentration with position can be measured in the humid helium case. This assumption is consistent with the minimum impact of the addition of 0.28% water vapor to the helium feed gas on the plasma deposited energy and gas temperature (see further). Even though minor changes occur in the ionization wave propagation speed (see further), the impact on the flow dynamics will be due to surface charging rather than the actual ionization wave for our experimental conditions [41]. We anticipate that even minor changes in surface charge density are anticipated to relax towards a similar surface charge density on the timescales relevant for the gas dynamics.
The large variation in OH(A) lifetimes also requires us to make an accurate correction for the delayed 15 ns LIF signal collection. This correction was implemented by determining The corrected LIF signal for the 15 ns delay is also shown. The signals were collected for the dry helium case at a distance of 3.5 mm from the quartz substrate-capillary contact point (see figure 1). In this case, the LIF signal for 3600 laser pulses was collected to limit the laser scattering on the surface at 0 ns while 5940 accumulations were used for the measurement with 15 ns delay. the ratio of the time integrated LIF signal with and without 15 ns delay. This correction factor is shown in figure 3(a) and drops down to 0.1 for 10% air, meaning that only 10% of the total fluorescence is detected with a 15 ns delay in the ICCD gate compared to the laser pulse. A similar calculation was made for the humid helium case. Since the water vapor increase to ∼0.28% in helium, with a 6% air admixture (which is the largest amount considered in this work), the lifetime of OH(A) will reduce to 7.0 ns and the corresponding correction factor is 0.069. The measurement of OH LIF for significant larger concentrations for the approach reported in this work might require a laser with a shorter pulse duration. Figure 3(b) shows an example of the OH(A) lifetime and LIF intensity measurement as a function of distance from the quartz surface. The LIF signals both with and without 15 ns delay are compared. In this specific case, a detectable LIF signal was observed up to a distance of 800 µm from the substrate. The comparison of the LIF intensity recorded with and without delay shows that the delayed measurement is much smoother and leads to a significantly smaller LIF signal near the substrate which is due to the removal of the laser scattering near the substrate and the ability to increase the number of accumulations on the ICCD without saturation. Even for the 7 ns OH(A) lifetime, we had sufficient signal-to-noise ratio to enable between 3 to 8 delay measurements and an accurate fit of the fluorescence decay to determine the OH(A) lifetime although the uncertainty became significant for the lower end of the determined lifetimes as can be seen in figure 3(b).

Plasma dynamics
Before describing the results from the OH LIF study, we describe the plasma conditions and dynamics. The plasma discharge conditions are very similar as reported in the work of van Doremaele et al [42]. Figure 4 shows the voltage, conduction current and power waveforms for the dry helium discharge case. The conduction current was obtained by subtracting the displacement current (obtained from a measurement of the same voltage pulse although without plasma formation) from the measured discharge current. The plasma dissipated power was calculated by multiplying the obtained current and voltage waveforms. However, the current peak at the falling edge of the voltage pulse occurs after the voltage dropped down significantly. The reignition of the discharge in this case is due to the remaining deposited charge on the dielectric substrate. We estimated in this case the upper limit of the plasma dissipated power by assuming that the space charge field immediately after the voltage pulse results in the same voltage amplitude as the voltage pulse although with opposite sign. This approach was used before [31] for a plasma jet and yielded good agreement between measured and predicted radical densities as obtained with 0D modeling (using the experimental power waveforms as input).
In the dry case the energy deposited during the voltage rise is 11 µJ while the energy at the falling edge will remain smaller than 22 µJ yielding a maximum energy per pulse of 33 µJ. The humid case yields the same result (not shown). This low energy deposition allows us to assume that the gas temperature remains at room temperature [31]. This was confirmed by a gas temperature measurement at a distance of 1 mm from the nozzle with OH LIF as described in the method section yielding a value of 306 ± 49 K.
The plasma dynamics during the entire voltage pulse can be seen in the images shown in figure 5 for both the dry and humid helium case. The highest emission intensity was found during the voltage pulse rise and is caused by the formation and propagation of a surface ionization wave [43]. When the voltage is switched off, a reillumination of the plasma channel was found due to the field reversal sustained by surface charge deposited on the quartz substrate and possible the quartz capillary as in a dielectric barrier discharge. This effect has also been previously reported in an argon pulsed plasma jet showing an increase in electron density and temperature during such reignition [44].
The addition of water vapor to the helium emission reduces the plasma intensity. A distinctive difference between the two cases is that upon water addition to the helium gas feed a stronger emission near the jet nozzle is found in the second half of the voltage pulse likely due to additional emission sources such as OH and H in the presence of H 2 O. In both cases, the thickness of the surface ionization wave reduces with increasing distance from the jet nozzle due to the increasing air diffusion into the helium jet (see also further). Figure 6 shows the propagation of the surface ionization wave during the raising and the falling edge of the pulse. These images allow the estimation of the propagation speeds of the ionization waves for both cases. For both the rising and falling edge, the dry helium case has the highest propagation speed of about 2.0 ± 1.3 × 10 5 m s −1 near the nozzle which reduces with increasing distance from the nozzle to a value of 0.71 ± 0.06 × 10 5 m s −1 . The humid helium case, however, has a relatively constant propagation speed of 1.8 ± 0.5 × 10 5 m s −1 within the investigated region. A significant reduction in propagation speed with increasing distance from the nozzle has also been observed for the free jet [45] and can be explained by increasing air concentrations for increasing distances. The increased air concentration reduces the Townsend ionization coefficient compared to pure helium at a fixed electric field. The presence of 0.28% water vapor in the jet increases initially the ionization rate due to the lower ionization energy of water compared to helium and also likely reduces the impact of increased air concentrations during the propagation of the ionization wave, at least initially.

Air concentration
The air concentrations, measured as described in the method section, 10 µs after the discharge pulse ends, at three vertical positions are shown in figure 7. The air concentration at a distance of 1 mm from the nozzle, remains low (0.2%) and significant mixing of air was only found for distances in excess of 650 µm from the quartz substrate. The air concentration at the quartz substrate increases with increasing distance from the nozzle reaching 2% at 5.2 mm. This enhanced concentration is consistent with the found decrease in propagation velocity of the ionization wave for the dry helium case. Indeed, the Townsend ionization coefficient starts to reduce upon addition of an air concentration in excess of 1% [46]. At 3.5 mm, the air concentration already starts to increase at a distance of 200 µm from the quartz substrate, consistent with the thinner surface ionization wave observed at this location compared to the ionization wave at the position of the nozzle (figure 5).

OH and water vapor density distributions
The OH and water vapor densities near the quartz substrate were recorded at three different vertical positions for both the  dry and humid helium case as shown in figure 8. Even in the case of dry helium, a significant amount of water vapor is present near the nozzle (∼5 × 10 14 cm −3 or 20 ppm) caused by the diffusion of the ambient air containing 1% humidity into the jet effluent. The water vapor density near the substrate has a similarly increasing trend as the air concentration (figure 7) and reaches a value of ∼5 × 10 15 cm −3 or 200 ppm at a distance of 5.2 mm from the nozzle. In the humid helium case, the water vapor density is significantly larger and is dominated by the admixed water vapor to the helium feed gas yielding a water vapor density of 1.0 ± 0.3 × 10 17 cm −3 . The admixed H 2 O to the helium feed was 0.28 ± 0.07% (see method section) corresponding to 6.7 ± 1.6 × 10 16 cm −3 and is within the experimental accuracy of the measured water vapor density. The significant difference in water density near the substrate for the dry and humid helium case leads to an OH density that is one order of magnitude larger for the humid helium case compared to the dry helium case. The OH density increases, similar to the H 2 O density, with increasing distance from the plasma jet nozzle. A significant gradient in the OH density is found near the quartz surface at larger distances from the quartz nozzle with density variations of up to two orders of magnitude over 500 µm. This gradient is more pronounced for the dry compared to the humid helium case. The emission intensity including the OH emission is not strongly correlated with the local OH or H 2 O density. Interestingly, the maximum OH density occurs Figure 8. The OH density, water vapor density, plasma emission and OH emission intensity as a function of the distance from the quartz substrate at three different vertical positions for the dry and humid helium case. (a) 1 mm dry helium, (b) 1 mm humid helium, (c) 3.5 mm dry helium, (d) 3.5 mm humid helium, (e) 5.2 mm dry helium and (f) 5.2 mm humid helium. The plasma emission and OH emission intensity (captured with a bandpass filter) are all normalized to the dry helium case for each vertical position to facilitate comparison. Note that the reported water vapor density is reduced by a factor 100 or 1000 for dry and humid helium, respectively (as indicated in the figure) to minimize the density range in the figures. All LIF measurements were performed 10 µs after the discharge pulse. mostly at a larger distance from the quartz substrate than the maximum in emission intensity. While the emission intensity is representative for the position of the ionization wave, the large gradients in air densities cause an increased collisional quenching of excited states with increasing distance from the discharge which might underestimate the thickness of the ionization zone from emission. If we assume that the gradient in emission intensity near the quartz substrate is representative for the sheath structure between the ionization wave and the quartz substrate, a reduction in thickness of the sheath is observed with increasing distance. The decrease in sheath thickness with increasing distance from the nozzle, which is  0.60 ± 0.14 particularly clear for the dry helium case, does however not seem to lead to a stronger gradient in the OH density near the substrate.
The OH densities as a function of the distance from the quartz surface for the dry helium and the humid helium cases at different delay times compared to the start of the voltage pulse are shown in figure 9. The spatial OH density distributions are very similar for each time point. In the dry helium case, the overall OH density decreases with increasing delay time allowing to calculate the (effective) OH radical lifetime. Nonetheless, in the humid case, an increase in OH density at 700 µs was observed. This is likely due to a set of reactions involving OH, O and HO 2 radicals that have been previously shown to enhance OH production in the afterglow [47]. Table 1 lists the estimated OH radical lifetime at the quartz surface and at the position of the maximum OH density for both the dry and humid helium case. The typical diffusion time, τ d can be estimated as follows with l the diffusion length scale and D the diffusion coefficient taken from [48]. Considering a typical length scale of 400 µm in this work (see figure 8), yield a time scale of 32 µs which is more than an order of magnitude smaller than the effective lifetime of the OH radical. While the OH recombination timescale due to radical-radical recombination can reach ∼600 µs in the bulk of the plasma as estimated by the approach used in [38], the recombination time is 1-10 ms near the substrate due to the lower OH densities and hence diffusion will be the dominant loss near the quartz substrate. Hence, the OH gradient is strongly impacted by the OH loss probability at the quartz wall.

OH flux and loss probability coefficient
The OH density gradient near the quartz substrate as shown in figure 8 allows calculating the OH flux at the position of the quartz surface. Figure 10 shows the OH radical diffusive flux at the substrate as calculated by the OH density gradient over the first 100 µm distance from the quartz substrate. The OH density in the first 100 µm distance can in good approximation be considered linear for all investigated conditions of figure 9. The OH flux has a maximum value of ∼1 × 10 15 cm −2 s −1 and decreases two orders of magnitude within 2.4 ms (figure 10). Yonemori and Ono reported the OH flux in a plasma jet impinging on a quartz surface for similar conditions and found a similar value [49]. The thermal OH flux, 1 4 nv th at the interface can also be determined from the OH density at the quartz substrate and the thermal velocity, v th of the OH molecule at room temperature. The thermal OH flux has the same trend as the diffusive OH flux although is ∼100 times lower. The diffusive flux, Γ d is related to the thermal flux as follows [17]: with β the overall surface loss probability of the OH radical on the quartz substrate. The constant ratio of the thermal and diffusive flux yields a constant surface loss probability of 0.012 ± 0.008 for the dry quartz surface (figure 10). Table 2 shows the calculated OH, diffusive flux, thermal flux and loss probability for both the dry and humid helium cases at the three positions reported in figure 8. For the dry   Table 3. Impact of the spatial resolution of the OH measurement on the OH density, diffusive flux, thermal flux and loss probability determination for the dry helium case at 3.5 mm from the nozzle (figure 7(c)).
No loss probability was reported for the 1 mm case in humid helium because of the lack of gradient in the OH density near the substrate. The loss probability of OH on quartz surfaces has been measured at low pressure [21,50] and at high-pressure and elevated temperature [34,35]. At low pressure, the reported β was reported to be ∼10 −3 . However, in an atmospheric combustion environment, the value of β increased to 0.01 similarly to the measured value in a room temperature plasma. It is hard to draw more conclusions from this comparison as the loss probability coefficient could depend on the properties of the quartz substrate used. Nonetheless, the constant loss probability for a variation of almost two orders of magnitude (figure 10 and table 2) for the dry helium case suggests that a constant β coefficient might be representative for near room temperature atmospheric pressure plasmas.
The determination of the OH flux to the substrate can be impacted by the spatial resolution of the measurement as shown in table 2. If we average five data points to achieve a 50 µm resolution (most LIF measurements have similar or even lower resolution), we have only four density measurements over 200 µm. The comparison between 10 and 50 µm resolution shows an overestimation of the diffusive flux and loss probability by a factor of ∼2. Note that the spatial resolution is mainly important for the determination of the diffusive flux. As one can see in figure 8, it is critical to determine the slope from data points within 100 µm of the substrate as for larger distances the slope tends to change. The fact that the ratio of the diffusion flux and thermal flux is constant for the range of conditions studied in this work (approximately two orders of variation in density) suggests a resolution of the order of 10 µm is satisfactory for near substrate measurements. This is further supported by the result shown in table 3 for a spatial resolution of 50 µm.

Conclusion
In this work, we reported the OH density distribution in a surface ionization wave in helium in a surrounding air environment on a quartz substrate as measured by microscopic LIF. In addition, the OH (A) fluorescence lifetime was used to determine the air and water vapor density distributions in the plasma jet effluent. A significant gradient of the OH density near the quartz substrate was found with OH densities that are small enough so the dominant OH loss mechanism near the substrate are diffusive fluxes to the substrate. This is confirmed by a constant ratio of the diffusive and thermal OH flux to the substrate allowing us to derive a surface loss probability for the quartz surface of 0.01. The surface loss probability is an important parameter for the modeling of plasma surface interactions which was to our knowledge not yet reported for room temperature atmospheric pressure plasma conditions. Moreover, this work shows that high spatial resolution of the order of 10 µm is required to allow for accurate measurements of the OH density and flux near substrates in surface discharges.

Data availability statement
The data that support the findings of this study are available upon reasonable request from the authors.