Hydroxyl Radical Fluorescence and Quantum Yield Following Lyman

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A: Spectroscopy, Molecular Structure, and Quantum Chemistry

Hydroxyl Radical Fluorescence and Quantum Yield Following Lyman-alpha Photoexcitation of Water Vapor in a Room Temperature Cell and Cooled in a Supersonic Expansion Justin W. Young, Ryan S. Booth, Kristen M. Vogelhuber, Jaime A Stearns, and Christopher J. Annesley J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.8b03047 • Publication Date (Web): 05 Jun 2018 Downloaded from http://pubs.acs.org on June 5, 2018

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The Journal of Physical Chemistry

Title: Hydroxyl Radical Fluorescence and Quantum Yield Following Lyman-alpha Photoexcitation of Water Vapor in a Room Temperature Cell and Cooled in a Supersonic Expansion. Authors: Justin W. Young†,‡, Ryan S. Booth†,‡, Kristen M. Vogelhuber†,‡,¥, Jaime A. Stearns†, Christopher J. Annesley*,† Author Addresses: †

‡

Space Vehicles Directorate, Air Force Research Laboratory, Kirtland AFB, New Mexico 87117, USA Boston College, Institute for Scientific Research, Chestnut Hill, Massachusetts 02467, USA

Corresponding Author: *C. J. Annesley E-mail: [email protected] Abstract Photoexcitation of water by Lyman-alpha (121.6 nm) induces a dissociation reaction that produces OH(A2Σ+) + H. Despite this reaction being part of numerous studies, a combined understanding of the product and fluorescence yields is still lacking. Here, the rotational and vibrational distributions of OH(A) are determined from dispersed fluorescence following photoexcitation of both room-temperature and jet-cooled water vapor, for the first time in the same experiment. This work compares new data of state-resolved fluorescence with literature molecular branching ratios and brings previous studies into agreement through careful consideration of OH(A) fluorescent and predissociation lifetimes, and confirms a fluorescent quantum yield of 8%.

Comparison of the room temperature and jet-cooled OH(A)

populations indicating the temperature of H2O prior to excitation has subtle effects on the OH(A) population distribution, such as altering the rotational distribution in the ν′ = 0 population and affecting the population in the ν′ = 1 state. These results indicate jet-cooled water vapor may have a 1% higher fluorescence quantum yield compared to room-temperature water vapor.

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Introduction Due to the ubiquitous presence of water, its photodissociation is an important process in environments where vacuum ultraviolet (VUV) light is present.

In the terrestrial mesosphere, the

photodissociation products of water are a major contributor to the environment’s O atom and ozone chemistry.1 Water is a common molecule in deep space, and observations of water are combined with knowledge of its photochemistry to gauge characteristics of interstellar environments.2 Furthermore, water is a principal molecule in cometary atmospheres where thermalization of water’s energetic photodissociation products contributes to heating comets’ atmospheres.3 Finally, spectra of the Space Shuttle engine plume show OH emission, and subsequent analysis had to take into account water photodissociation to properly describe the plume spectrum, demonstrating the process’s importance in space vehicle plumes.4 The understanding of electronic excitation and subsequent photodissociation of water with Lyman-alpha radiation, 121.6 nm, carries particular significance because of the particular intensity of this line in the solar spectrum.5 The photodissociation of water has been studied extensively for decades, starting with the observation of fluorescence from hydroxyl radicals in the A2Σ+ state following the photolysis of water with VUV radiation,6 and has since been studied with a variety of techniques and calculations.7-21 (1A1) state of water, centered at 122 nm. This state is Excitation of water with Lyman-alpha excites the short lived and quickly dissociates through the (1A1) state of water due to an avoided crossing between (1A1) and (1A1) electronic surfaces.10,15,19,21 The energetically allowed products from this process the are shown below: ) H2O(

→

H2O( ) →

OH(X) + H

(1)

→

OH(A) + H

(2)

→

O(1D) + H2

(3)

→

O(3P) + 2H

(4)


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Dissociation from the (1A1) surface to these products has been studied previously and has been shown to be intricate and influences the internal energy of the products. For instance, this surface has two conical intersection which can produce a quantum interference effect resulting in an alternating intensity pattern for OH products with even and odd rotational states.12,14 Furthermore, the internal energy of the OH(A) products from the (1A1) surface are quite sensitive to the excitation energy due to transient (1A1) and (1A1) states, the initially resonances.13,20 With the rapid electronic coupling between the populated water states will affect the resulting OH(A) rotational and vibrational population. The molecular branching ratios of the four channels have been the focus of several studies. Slanger and Black measured the product yields to determine the molecular branching ratios: 78% for products (1)+(2), 10% for channel (3), and 12% for channel (4).8 Later, Mordaunt et al. used the translational H-atom Rydberg-tagging time-of-flight (HRTOF) technique to measure the relative branching ratios for processes (1), (2) & (4).11 Therefore, Mordaunt et al. combined their results with those of Slanger and Black to obtain the absolute molecular branching ratios for all four channels: (1) 64%, (2) 14%, (3) 11%, and (4) 11%. Another HRTOF experiment was performed with higher resolution by Harich et al., who determined the relative branching ratios (1) 66%, (2) 13%, and (4) 21%, additionally, they presented quasiclassical trajectory calculations suggesting that channel (3) is a minor channel.14 Of these channels, channel (2) uniquely produces a fluorescent product, OH(A). Carrington reported one of the earliest estimates of the fluorescence quantum yield to be 5%, within a factor of 2 or 3.7 Later the value was quantified by Vinogradov and Vilesov as 5.2 ± 0.6%22 and 5.2 ± 1%.23 A higher quantum yield was reported by Lee, who measured the value as 8 ± 2%.24 A similar measurement was reported by Dutuit et al., who determined yields of 3.7% and 7.2% for the nearby wavelengths 120.6 and 121.9 nm.25 Discrepancies between values reported by Dutuit et al. and Lee, led Lee and Suto to repeat the quantum yield measurements, and the results agreed with Lee’s earlier measurement of 8%.26 These results are summarized in Table 1.



Table 1. OH(A) Quantum Yields Due to the Lyman-alpha Photolysis of Water Vapor. Reference Measurement Value Error Carrington7 Φf 5% +15/-3 Vinogradov and Vilesov22 Φf 5.2% ±0.6 Vinogradov and Vilesov23 Φf 5.2% ±1 Lee24 Φf 8% ±2 Dutuit et al.25,a Φf 7.2% ±0.7 Lee and Suto26 Φf 8% ±2 b Mordaunt et al.11 Φm 14% 14 b Harich et al. Φm 13% Φf indicates fluorescence quantum yield from, Φm indicates OH(A) molecular branching ratio. a

121.9 nm excitation.

b

Value not reported.

Accurate quantities of the branching ratio into channel (2) and its fluorescence are particularly necessary for astronomical models because this is the only channel that can be detected remotely. However, due to the deviation in measurements, shown in Table 1, some astronomers describe the fluorescence quantum yield as not accurately known.3

Furthermore, a lack of consensus is also

demonstrated by some authors opting to use the molecular branching ratio of channel (2) to describe the fluorescence quantum yield of water photolysis;4 as reports of this quantity are more recent and have been consistently measured, 13% – 14%.11,14 However, these measurements are higher than the measured fluorescence quantum yields (Table 1), and may not reflect the fluorescence. Thus, there is clear need to reinvestigate this system, in particular the relationship between branching into the OH(A) channel and its subsequent fluorescence. There is a lack of knowledge of the effect of temperature on water photolysis; the HRTOF experiments were performed in molecular beams,11,14 and fluorescent measurements have almost entirely been performed at room temperature.7,22,24,26 To the best of our knowledge, Vinogradov and Vilesov preformed the only study measuring the effect of temperature on the formation of OH(A) and its fluorescence, where the dispersed fluorescence spectrum and fluorescence quantum yield were measured at two temperatures, 25 oC and 300 oC.23 Their results show heating the water vapor causes large changes in the OH(A) rovibrational structure, and a small decrease in fluorescence quantum yield, less than their 4 ACS Paragon Plus Environment

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experimental error (20% of 5.2%). The effect of temperature and the relation between all of these measurements merits further study to form a complete, quantified picture of VUV water photolysis. Here, we investigate the OH(A) photolysis product’s rovibrational state distribution using dispersed fluorescence and establish the connection between the fluorescence spectra and fluorescence quantum yield measurements with HRTOF measurements. Furthermore, we perform these experiments at both room temperature and cooled in a supersonic jet expansion in order to begin to understand the effects of temperature. Experimental and Analysis Apparatus The results presented in this work were obtained using the apparatus shown in Figure 1. Tunable VUV radiation is generated through a difference-frequency four-wave mixing scheme (2ω1-ω2). To accomplish this a UV laser beam is tuned to a resonant two-photon allowed transition of krypton (212.5 nm), ω1. The 212.5-nm UV beam is created with the tripled output of a 10 Hz Nd:YAG (Spectra Physics PRO-230-10) pumped dye laser (Lambda Physik FL3002). This dye laser produces roughly 25 mJ/pulse of 637-nm light. The output is doubled through a KDP nonlinear crystal, rotated with a zero-order halfwave plate and then sum-frequency mixed with the fundamental beam using a BBO crystal, effectively tripling the output from the dye laser. The tripled output, ~1 mJ/pulse of 212.5-nm light, is separated from the doubled and fundamental beams using a Pellin-Broca prism. A second Nd:YAG (Continuum Powerlight 7000 series) pumped dye laser (Lambda Physik FL3002) is used to generate an 845-nm beam with energies close to 8 mJ/pulse. The UV and IR beam are spatially overlaid using a dichroic mirror, reflective for 212 nm.



Figure 1. The experimental apparatus for observing dispersed fluorescence following photoexcitation from vacuum ultraviolet light, specifically 121.6 nm. For VUV generation, the collinear beams are directed into a krypton gas cell. For proper phase matching, the gas cell contains a mixture of krypton and argon gas, where the argon acts as a positive index matching medium.27 The cell’s length is 30 cm and its entrance window is a focusing lens with a focal length of 20 cm. Chromatism created by this lens is corrected with two lenses inserted into the IR beam path before the two beams are made collinear to ensure the UV and IR beam waists overlap. A second 10-cm focal length MgF2 lens serves as the seal between the gas cell and experiment chamber. This lens filters out the sum frequency VUV light (2ω1+ω2), and approximately recollimates, the beams into the 50 cm experiment chamber where they pass in front of collection optics centered in the chamber, and then to a baffle arm and exit window. Fluorescence is collected using a 4-cm focusing lens, and the image is refocused onto the slit of a spectrograph (Acton Research Corp. Spectrapro-150) with a 15-cm focal length and an 1800-grooves/mm grating (Acton Research Corp. 150-180-250). The dispersed fluorescence is detected using an intensified CCD camera (Princeton Instruments PI-MAX). With this setup a slit width of 80 µm achieves a resolution of 0.35 nm. A spectral range of 70 nm was used, allowing for both the ∆ν = -1 and the ∆ν = 0 portions of the spectrum to be recorded simultaneously, and the excitation laser beams are outside of the spectrograph’s acceptance window. Here, a gating time of 1500 ns, encompassing the known lifetimes of


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these OH(A) states28 was used. The data was accumulated on the CCD camera for 1,200-shot increments and after every 10 increments a new 1,200-shot background was recorded. The final spectra were the accumulation of about 400,000 shots. The spectral response of the camera was determined with a calibrated deuterium lamp (StellarNet SL3). Measurements of room-temperature water vapor were performed by establishing a flow through the chamber pumped by a mechanical dry pump and continuously replenishing the vapor to maintain a pressure of 80 mtorr with a needle valve, as measured by a Baratron capacitance manometer (MKS, 310UHS-100). At this pressure, Carrington reported that collisions with water do not affect the rotational distribution of OH(A).7 Jet-cooled water vapor measurements were performed by expanding a pulse of helium seeded with water vapor at a backing pressure of 1030 torr through a 0.5-mm orifice of a pulsed solenoid valve (Parker Hannifin : Pulse Valve Series 9) into the same vacuum chamber, now pumped by a turbomolecular pump (Agilent Varian TV551 Navigator). For these measurements the water reservoir and solenoid valve were wrapped in heating tape and kept at ~80 oC, using a Variac to maintain temperature and monitored using a thermocouple gauge. This corresponds to 350 torr of vapor pressure and ~34% beam of water in helium. One possible effect of cooling could be the production of water dimers or higher order clusters. Water dimers are not expected to influence the room temperature spectrum,29 and we see no clear new features in the jet-cooled fluorescence spectrum to attribute to photolysis products from water dimers. We have also measured the jet-cooled spectrum using a ~60 oC nozzle/reservoir with the same backing pressure to halve the water vapor concentration in the jetexpansion, and thus reduce the formation of water dimers. There is no significant difference when comparing the two jet-cooled spectra demonstrating that dimers are not influencing the spectra. Spectral Deconvolution The procedure used to analyze the emission spectra is modeled after previous efforts to determine the rovibrational populations from the partially resolved emission spectra.16,17,30 The transitions of OH(A) are modeled to have a Gaussian lineshape with a width determined by the spectrometer, 0.35 nm, and a peak intensity determined by the equation: 7 ACS Paragon Plus Environment


Iν′,N′,ν′′N′′ = nν′,N′Aν′,N’,ν′′,N′′ (5) where the Einstein coefficients, Aν′,N′,ν′′,N′′, and transition frequencies of OH(A-X) are from LIFBASE assuming Hund’s case (b) selection rules,31,32 and nν′,N′ are the unknown populations. Similar to previous work,7,16,17 the F1 and F2 populations are set to equality for like N′, as high resolution studies of OH(A) have shown these two levels contain equal populations. Only OH(A) levels up to ν′ = 0 N′ = 25, ν′ = 1 N′ = 20, and ν′ = 2 N′ = 15 are used since higher levels are unobserved. To find the excited state populations, the spectrum is divided into wavelength segments defined by the experimental bins (e.g. each pixel in the experimental data), and fit with equation (6): λ

Ibin = ∑ν' ,N' (nν',N' ∑ν'' ,N'' Aν' ,N',ν'' ,N'' λ 2 gν',N' ,ν'' ,N'' (λ)dλ) (6). 1

The experimental intensity of each bin, Ibin, is set equal to the sum of intensities reflecting all of the unknown excited state populations’, nν′,N′, individual contribution within the bin’s spectral range, λ1 – λ2. Gaussian profiles, g ν′,N′,ν′′,N′′ (λ), represent each transition, as described above. Expressing all of the bins of data with equation (6) creates an overdetermined system of linear equations that can be solved for nν′,N′ through standard minimization procedures. We have elected to minimize equation (6) using a nonnegative least squares method because it produces values held to the physical constraint that negative populations have no physical meaning. This method does have the drawback that there is no general method to assign errors in the fit, but based on signal to noise and multiple data fits, we estimate an error of a few percent in population. However, it should be noted that other efforts to analyze a partially resolved spectrum of OH(A) opted to use a truncated singular value decomposition (TSVD) procedure to minimize equation (6).16,17,30 Results The dispersed fluorescence spectrum arising from the photolysis of water in a cell at 22 oC using 121.6nm light as the excitation source is shown as the black trace in Figure 2. The observed fluorescence originates from the photolysis product OH(A). The spectrum displays two predominant features centered around 285 and 315 nm. These two progressions are associated with ∆ν = -1 and ∆ν = 0 transitions of the


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electronic OH(A→X) transition, respectively. The ∆ν = 0 progression spans 20 nm and possesses partially resolved features from rotational transitions of the OH(A) product. The ∆ν = -1 progression is noticeably less intense than the ∆ν = 0 transition and also lacks the appearance of any resolved rotational features. The progression spans 16 nm and possesses a tail of low intensity transitions on its red side. Fluorescent transitions associated with other ∆ν are not observed in this experiment as they are lower transition

probability

and

contain

redundant

information.

Figure 2. The dispersed fluorescence spectrum (top, black) resulting from photoexcitation of 22 OC water vapor with 121.6 nm light. The fluorescence observed here is a result of the photodissociation product OH(A). Simulated spectra (red) are deduced from a rovibrational population analysis of OH(A).

The features of the spectrum are well reproduced by the simulated spectrum (red) shown in Figure 2. The simulation total is shown and has furthermore been divided into vibrational contributions to demonstrate each level’s relative significance to the spectrum. The populations that produced the simulated spectrum are shown in Figure 3. Population was observed in levels ν′ = 0, 1, 2 with most of the OH(A) population found in level ν′ = 0. The population distribution in this level has an inverted appearance with over half of the ν′ = 0 population found in rotational levels N′ = 15 – 22. The population found in ν′ = 1 does not have the same heavily inverted appearance in ν′ = 0. While still a non-Boltzman distribution, the



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population has a more equal distribution, and reaches up to level N′ = 18. Only a small portion of the total population was observed in the ν′ = 2 level. Furthermore, the low intensity of the ν′ = 2 transitions, Figure 2, approaches the noise level of the experiment, and could cause an over estimation of this population. The vibrational states are observed to have the relative populations 75%, 22% and 2% for states ν′ = 0, 1, and 2, respectively. Based on the analysis’ sensitivity to noise, these values are estimated to

be

accurate

within

a

few

percent.

Figure 3. The rovibrational populations of OH(A) following photodissociation of water with 121.6 nm light as observed from dispersed fluorescence of room temperature water vapor (red), and jet-cooled water vapor (blue). This figure uses the same scale as Figure 5.

The fluorescence spectrum recorded by seeding water in a jet-cooled supersonic expansion of helium is shown in Figure 4. This technique is known to cool samples well below room temperatures,33 although it can be challenging to obtain rotational and vibrational temperatures of the beam. The subtle 10 ACS Paragon Plus Environment

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differences between the room temperature (red) and jet-cooled (blue) measurements can be observed in the spectral contours, as shown in Figure 4. For instance, in the ∆ν = 0 region near 315 nm, each spectrum possesses different prominent peaks. Furthermore, the jet-cooled spectrum has more intensity in the ∆ν = -1 region, near 285 nm, than the room temperature spectrum. Analysis of the dispersed fluorescence spectrum provides the relative rovibrational populations of OH(A), which are shown in Figure 3. The relative vibrational distributions obtained in the jet-cooled spectrum in ν′ = 0, 1, and 2 are 68%, 28%, and 4%, respectively. The error of these values is estimated to be within a few percent.

Figure 4. The dispersed fluorescence spectrum resulting from photoexcitation of water vapor at room temperature (red) or seeded into a supersonic jet expansion (blue) with 121.6 nm light. The fluorescence observed here is a result of the photodissociation product OH(A).

Discussion The

OH(A)

rovibrational

populations

derived

here

from

room

temperature

water

photodissociation are similar to those from previous fluorescent studies on the photolysis of H2O.7,16,17,23 The rotational contour presented here looks similar to what was presented by Vinogradov and Vilesov at 25 oC.23 Furthermore, Carrington measured the emission spectrum of water excited with Lyman-alpha at higher resolution, 0.035 nm, and reported a rotational distribution much like what we report in Figure 3, and a similar vibrational distribution as well, see Table 2.7 11 ACS Paragon Plus Environment

The good agreement between our


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rovibrational populations with Carrington’s suggests that the resolution and subsequent analysis implemented here provide a reasonable representation of the fluorescent hydroxyl radical populations. Table 2. Observed OH(A) Vibrational Distributions Following Photodissociation. Reference Temp. v′ = 0 1 2 This Work 22 oC Φf 75 23 2 This Work Jet-Cooled Φf 68 28 4 Carrington7 Room Temp. Φf 77 23 0. In state ν′ = 1, the effect is more subtle, where the prominent population near N′ = 17 and 18 has been greatly reduced, while the population in lower states remains unchanged. Moreover, this adjustment improves the agreement with the populations deduced from the jet-cooled fluorescence recorded here, showing this adjusted population is effectively a fluorescent population. However, some differences still remain even after adjusting the HRTOF population.

The ν′ = 1

population total is greater in the adjusted HRTOF results than the fluorescence measurements, and the ν′ = 0 populations shows some subtle differences in the most prominent populations. These disagreements likely result from the temperature difference between the experiments, as the HRTOF are taken at 10 K.14



Figure 5. The rovibrational populations of OH(A) following photodissociation of water with 121.6 nm light as observed from dispersed fluorescence of jet-cooled water vapor (blue), this work, and HRTOF energy measured by Harich et al (gold).14 The effective luminescence (black dots) is determined by multiplying the populations deduced from Harich’s previous HRTOF results by the lifetime ratio, (τ/τR). The OH(A) ν′ = 0 population has been set to equivalence, and the sum of the HRTOF population is equal to 100. Applying equation (8) to the HRTOF derived OH(A) population decreases it by 28%, or alternatively stated, 72% of the OH(A) population is fluorescent, as shown by the difference between the gold and black markers in Figure 5. We can use this information to estimate the fluorescence quantum yield of channel (2) with the molecular branching ratio of channel (2), 13%,14 by multiplying the molecular branching ratio value by the percentage of fluorescent population, thus producing an estimate of the fluorescent yield, 9.3%. Similarly, the fluorescence quantum yield may also be estimated from the population distribution deduced from the fluorescence measurements recorded in this work via direct comparison to the HRTOF results from Harich et al. to predict how much of the population is fluorescent.14

To achieve this, the sum of populations in the ν′ = 0 states, which possess little

predissociation,34 are set equal. The remaining differences between the results expresses the difference


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between fluorescence and molecular yield caused by predissociation. This comparison estimates 69% of the OH(A) population is fluorescent, and 31% is predissociative. Given this prediction, the molecular branching ratio for the chemical equation (2) can again be used to estimate the fluorescence quantum yield, in this case 9.0%. The difference between this estimate and 9.3%, derived above, is likely due to small differences of the population in the ν′ = 1 rotational states, and to a lesser extent the ν′ = 2 states. There is limited knowledge of temperature effects on the photolysis of water, as most experimental techniques have only been applied to either room temperature or jet-cooled samples. This comparison is made with regard to the OH(A) population in Figure 3, where temperature has two subtle effects. First, changing temperature causes some differences in the rotational population distribution among rotational states in ν′ = 0, particularly between N′ = 18 – 23. These levels are affected by temperature despite mostly being below the photon energy, only the energy gap between OH(A, N′= 23) and the ground state of water exceeds the 121.6 nm photon energy and requires thermal energy to access. Second, cooling causes more population to be observed in the vibrational state OH(A, ν′ = 1), particularly in levels N′ = 17 and 18, as observed in Figure 3, and also Figure 6, where the OH(A) vibrational populations have been summarized. This observation is unusual since a colder sample would intuitively produce a colder vibrational distribution, meaning there is less population in OH(A, ν′ = 1). Simply, the increase in ν′ = 1 population could be caused by a cooling effect diverting population from the difficultto-observe high rovibrational states to lower, more visible, states in ν′ = 1, but this is difficult to confirm without the complete OH(A) distribution determined at room temperature. Fully explaining this behavior would require modeling the complicated nature of the (1A1) state, as described above, and is beyond the scope of this work. It is possible to use the population distributions to estimate the change in quantum yield of the room temperature water photolysis. This is obtained via direct comparison between the HRTOF results and our experimental data, as described above. This makes the assumption that the sum of population in ν′ = 0 and the channel (2) branching ratio do not change very much due to temperature. This assumption is supported by Figure 3 and Figure 5, where only small changes are noted in the distribution for ν′ = 0. 15 ACS Paragon Plus Environment


Figure 6 shows population detected in each of the three analyses we have discussed. With focus on the ν′ = 1 population, it can be seen the most fluorescence is expected for the coldest conditions, that of the predissociation adjusted HRTOF populations14 arising from 10-K molecular beam conditions (black triangles). Our free-jet expansion results have slightly lower fluorescent population (blue squares) that can be ascribed to a slightly warmer temperature, and finally the room temperature results (red circles) shows the smallest fluorescent population. This graphically shows the decrease in fluorescence with increasing temperature. The missing population between the fluorescent room temperature populations, and the HRTOF populations estimates the fluorescence quantum yield of channel (2) following room temperature excitation is 8.1%. This shows a 15% change in the fluorescence compared to our extracted 9.3% yield discussed above, giving the first comparison of the quantum yield with cold temperatures.

Figure 6. The vibrational populations of OH(A) following photodissociation of water with 121.6 nm light as observed from dispersed fluorescence of room temperature water vapor (▬●▬), jet-cooled water (▬■▬) or HRTOF results measured by Harich et al (▬▼▬).14 The effective luminescence (▬▲▬) is determined by multiplying the HRTOF populations by the lifetime ratio (τ/ τR). All populations have been normalized so that ν′ = 0 are equivalent, and the sum of the HRTOF populations is set equal to 100.


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Mordaunt et al.11 previously pointed out the related concern of possible temperature dependence of the molecular branching ratio. The mechanism of channel (1) involves Coriolis coupling, and thus increased rotational state population due to temperature may affect the branching ratios for all of the channels. This is a concern to the present work due to the comparisons made to an absolute fluorescence quantum yield only measured at room temperature and populations measured after cooling in a molecular beam. However, experimental observations show this effect seems quite limited when using Lymanalpha excitation. Vinogradov et al. measured the fluorescence quantum yield of the photolysis of water before and after heating their water sample to 300 oC and noted that with Lyman-alpha excitation, heating likely caused a small decrease, < 20%, of the room temperature fluorescence quantum yield value (5.2%).23 This experimental observation is qualitatively in agreement with our estimates and implies that the comparisons made above with room temperature and fluorescence yield is reasonable. Combining all available data, we estimate that heating from 10 K to 570 K will decrease the fluorescence quantum yield value by no more than 30%. All of the estimates, 8.1%, 9.0%, and 9.3%, of the fluorescence quantum yield fall close to and within the error of the direct fluorescence quantum yield measured by Lee et al, 8 ± 2%.24,26 This shows that the molecular branching ratios measured by Harich et al.14 and Mordaunt et al.11 are connected to the fluorescent quantum yield by the predissociation occurring in the OH(A) products, as predicted by the rates from Yarkony34 and verified by agreement with fluorescence results recorded here. Also, since the estimates for quantum yield decrease when warmer samples are used, this may indicate that fluorescence quantum yield decreases slightly with increasing temperature, due largely to the decrease in observable population of ν′ = 1.

Conclusion The photolysis of water by Lyman-alpha to produce OH(A) was reexamined and a connection between fluorescence yield and molecular branching ratio was established and the effect of temperature on the process was clarified.

When jet-cooled fluorescence results were compared to those from 17 ACS Paragon Plus Environment


HRTOF,14 it was apparent that the fluorescent results fail to detect a portion of radicals in higher rovibrational states. Applying an adjustment that accounts for OH(A) predissociation to the HRTOF largely resolves this disagreement. By adjusting the HRTOF population for predissociation and through comparisons to fluorescent populations, we predict that 62 – 72% of the product OH(A) population is fluorescent. Combining this value with the branching ratio for OH(A), 13%,14 we estimate the fluorescent yield is in the range of 9.3% at low jet-cooled temperatures and 8.1% for room temperature. This is in agreement with the room temperature measurement of fluorescent quantum yield from Lee and Suto, 8%.24,26 This provides evidence that fluorescence and OH(A) molecular yields are predictably connected by the predissociation that occurs in the OH(A) product. Temperature has subtle effects on the photolysis process. The rotational distribution of OH(A) in ν′ = 0 shows little change with temperature, and warming the sample shows a decrease in the relative amount of population in the ν′ = 1 states. Furthermore, this trend may cause the fluorescent quantum yield to decrease slightly with increasing temperature, with our estimates of yield changing from 9.3% to 8.1% when the temperature increases from 10 to 295 K. This discussion shows that there is a quantifiable connection between the fluorescent and molecular yield of OH(A) from water photolysis with Lyman-alpha excitation, and a small temperature dependence seems to be present. These concepts are important considerations for models concerned with OH(A) production and fluorescence, such as observations of space vehicle plumes as well as atmospheric and cometary environments.

Author Information Present Addresses: ¥

K. M. Vogelhuber: Space Dynamics Laboratory, 1695 Research Park Way, North Logan, UT 84341.

Notes

The authors declare no competing financial interests. 18 ACS Paragon Plus Environment

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Acknowledgments Funding for this work was provided by the Air Force Office of Scientific Research (LRIR #15RVCOR171, #18RVCOR121) under the Molecular Dynamics Program. The authors would like to thank Cheuk Ng and Zhou Lu for guidance with four-wave mixing. We thank Russell Cooper and Benjamin Prince for assisting with work related to this paper. We thank Rainer Dressler and Matthew Braunstein for useful discussions.



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(22) Vinogradov, I.; Vilesov, F. Luminescence of the OH (A2Σ+) Radical During Photolysis of Water Vapor by Vacuum UV Radiation. Opt. Spectrosc. 1976, 40, 32-34. (23) Vinogradov, I.; Vilesov, F. Effect of Temperature on the Process of Photodissociation of Water with Formation of the OH (A2Σ+) Radical. Opt. Spectrosc. 1978, 44, 653-655. (24) Lee, L. C. OH(A 2Σ+→X 2Πi) Yield from H2O Photodissociation in 1050–1370 Å. J. Chem. Phys. 1980, 72, 4334-4340. (25) Dutuit, O.; Tabche‐Fouhaile, A.; Nenner, I.; Frohlich, H.; Guyon, P. Photodissociation Processes of Water Vapor Below and Above the Ionization Potential. J. Chem. Phys. 1985, 83, 584-596. (26) Lee, L. C.; Suto, M. Quantitative Photoabsorption and Fluorescence Study of H2O and D2O at 50-190 nm. Chem. Phys. 1986, 110, 161-169. (27) Hilber, G.; Lago, A.; Wallenstein, R. Broadly Tunable Vacuum-Ultraviolet/ExtremeUltraviolet Radiation Generated by Resonant Third-Order Frequency Conversion in Krypton. J. Opt. Soc. Am. B 1987, 4, 1753-1764. (28) Brzozowski, J.; Erman, P.; Lyyra, M. Precision Estimates of the Predissociation Rates of the OH A 2Σ State ( v ≤ 2). Phys. Scr. 1978, 17, 507-511. (29) Tretyakov, M. Y.; Serov, E. A.; Koshelev, M. A.; Parshin, V. V.; Krupnov, A. F. Water Dimer Rotationally Resolved Millimeter-Wave Spectrum Observation at Room Temperature. Phys. Rev. Lett. 2013, 110, 093001. (30) Ruiz, J.; Martin, M. Application of the Truncated Singular Value Decomposition Method to the Obtention of Rovibrational Population Distributions from Electronic Spectra of Diatomic Molecules. Comput. Chem. 1995, 19, 417-431. (31) Luque, J.; Crosley, D. R. Transition Probabilities in the A 2Σ+−X 2Πi Electronic System of OH. J. Chem. Phys. 1998, 109, 439-448. (32) https://www.sri.com/engage/products-solutions/lifbase. (33) Smalley, R. E.; Wharton, L.; Levy, D. H. Molecular Optical Spectroscopy with Supersonic Beams and Jets. Acc. Chem. Res. 1977, 10, 139-145. (34) Yarkony, D. R. A Theoretical Treatment of the Predissociation of the Individual Rovibronic Levels of OH/OD (A 2Σ+). J. Chem. Phys. 1992, 97, 1838-1849.



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Figure 1. The experimental apparatus for observing dispersed fluorescence following photoexcitation from vacuum ultraviolet light, specifically 121.6 nm. 62x48mm (600 x 600 DPI)

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Figure 2. The dispersed fluorescence spectrum (top, black) resulting from photoexcitation of 22 OC water vapor with 121.6 nm light. The fluorescence observed here is a result of the photodissociation product OH(A). Simulated spectra (red) are deduced from a rovibrational population analysis of OH(A). 82x73mm (300 x 300 DPI)


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Figure 3. The rovibrational populations of OH(A) following photodissociation of water with 121.6 nm light as observed from dispersed fluorescence of room temperature water vapor (red), and jet-cooled water vapor (blue). This figure uses the same scale as Figure 5. 82x115mm (300 x 300 DPI)



Figure 4. The dispersed fluorescence spectrum resulting from photoexcitation of water vapor at room temperature (red) or seeded into a supersonic jet expansion (blue) with 121.6 nm light. The fluorescence observed here is a result of the photodissociation product OH(A). 82x68mm (300 x 300 DPI)


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Figure 5. The rovibrational populations of OH(A) following photodissociation of water with 121.6 nm light as observed from dispersed fluorescence of jet-cooled water vapor (blue), this work, and HRTOF energy measured by Harich et al (gold).14 The effective luminescence (black dots) is determined by multiplying the populations deduced from Harich’s previous HRTOF results by the lifetime ratio, (τ/τR). The OH(A) ν′ = 0 population has been set to equivalence, and the sum of the HRTOF population is equal to 100. 167x107mm (300 x 300 DPI)



Figure 6. The vibrational populations of OH(A) following photodissociation of water with 121.6 nm light as observed from dispersed fluorescence of room temperature water vapor (▬●▬), jet-cooled water (▬■▬) or HRTOF results measured by Harich et al (▬▼▬).14 The effective luminescence (▬▲▬) is determined by multiplying the HRTOF populations by the lifetime ratio (τ/ τR). All populations have been normalized so that ν′ = 0 are equivalent, and the sum of the HRTOF populations is set equal to 100. 82x73mm (300 x 300 DPI)


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Hydroxyl Radical Fluorescence and Quantum Yield Following Lyman

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