Direct Observation and Quantitative Measurement of OH Radical Desorption During the Ultraviolet Photolysis of Liquid Nonanoic Acid

Ultraviolet (UV) photolysis of fatty acid surfactants which cover the surfaces of atmospheric liquid aerosols and are found in the oceans such as nonanoic acid (NA) has recently been suggested as a source of hydroxyl (OH) radicals in the troposphere. We used laser-induced fluorescence to directly observe OH radicals desorbed from the surface of neat liquid NA as a primary photoproduct following 213 nm irradiation. The upper limit of photoreaction cross section for the OH radical desorption was estimated to be 9.0(4.1) $\times$ 10$^{-22}$ cm$^2$, which is only 1.2$\pm$0.8% of the photoreaction cross section established for the photolysis of gas-phase acetic acid monomers. Vibrational sum-frequency generation spectroscopy for liquid NA revealed the hydrogen-bonded, cyclic, dimer structure of the NA molecules at the liquid surface. This dimerization can inhibit the formation of OH radicals and lead the present low photochemical reactivity of liquid NA.

the intermolecular Norrish type-II reaction; the T1 C-C bond fission, generating octyl and carboxyl radicals, is the initiation process for the formation of C8 aldehydes. 7 Although the above experimental and theoretical studies advocate the importance of direct detection of primary photoproducts, especially OH radicals, to understand the photochemistry of NA, previous experimental approaches using mass spectrometry with continuous UV irradiation are not suitable for the detection of highly reactive species such as OH radicals. 2 Consequently, the mechanisms of the photochemical reactions of fatty acid surfactants such as NA remain poorly understood even in the neat liquid phase, let alone in a monolayer at the air-water interface. An alternative technique for detecting radical products is highly desirable to improve our fundamental understanding of the photochemistry of liquid fatty acids, which would eventually allow elucidation of their impact on the troposphere.
This study developed a new experimental apparatus for the UV laser photolysis of liquids by laser-induced fluorescence (LIF; Figs 1 and S2; see the Experimental Methods and Supporting Information for details). Our new approach enables direct detection of OH radicals photodesorbed from the surface of a liquid organic sample. As a first step, this study focused on the 213 nm photoexcitation of neat liquid NA via the S0-S1 transition, because liquid NA has a large absorption cross section of about 10 −19 cm 2 at around 210 nm ( Fig. S1 and ref. 8). We demonstrated that OH radicals are desorbed as a primary photoproduct following 213 nm excitation of liquid NA. We also quantified the photoreaction cross section of OH radical desorption by comparing the 213 nm photolyses of liquid NA and gas-phase acetic acid (AA).
Heterodyne-detected (HD) vibrational sum-frequency generation (VSFG) spectroscopy provided additional information about the mechanism of the photochemical reactions of liquid NA.
Considering both the LIF and HD-VSFG results enabled us to discuss the relationship between the surface structure and the photochemical reactivity of liquid NA. used a laser incident on the sample at 45° to its surface normal. (b) Gas-phase photolysis used two lasers crossing in a gas-filled chamber. Laser-induced fluorescence via the A 2 -X 2  transition of OH radicals was detected using a photomultiplier tube through a liquid light guide and two band-pass filters. z indicates the distance between the liquid surface and the center of the dye laser. d (= √2 ⁄ ) and d′ indicate the shortest distances between the centers of the two lasers.  Figure 2 shows normalized OH-LIF intensities with respect to delay time between the photolysis and probe lasers for the liquid-and gas-phase experiments illustrated in Fig. 1a and b (Figs S3 and S4 in the Supporting Information give further details of the data acquisition). The intensities during photolysis of liquid NA (red circles in Fig. 2) were measured using different detection points (3 mm ≤ z ≤ 7 mm), where z represents the distance between the liquid surface and the center of the dye laser (Fig. 1). The peak delay time for the OH-LIF intensity increased as z increased from 3 to 7 mm. This strongly suggests that the OH radicals desorbed from the liquid surface. Nevertheless, gaseous NA molecules were also present in the chamber, considering the vapor pressure of liquid NA (approximately 0.2 Pa) at room temperature. 9 Therefore, we separately conducted 213 nm photolysis of gas-phase NA using two crossed lasers (as shown in Fig. 1b) to evaluate the contribution of gas-phase photolysis of NA to the OH-LIF signals in the liquid experiment. The OH-LIF intensities from the gas-phase photolysis (black vertical bars in Fig. 2) were measured at different detection points (2 mm ≤ d′ ≤ 5 mm in Fig. 1b). The distance between the photolysis and probe lasers was approximately identical to that in the liquid-phase experiment (i.e., d ≈ d′ in Fig. 1a and b). The beam sizes and pulse energies of the two lasers in the gas-phase experiment were also identical to those in the liquid-phase experiment. When the two lasers were close to each other (d′ = 2 mm), an OH-LIF signal due to gas-phase NA photolysis was observed (Fig. 2). However, the OH radicals became barely detectable upon increasing d′ to 5 mm. The difference between the OH-LIF intensities measured in the liquid and gas phases should correspond to the amount of OH radicals desorbed from the liquid NA, which was much larger than that from the gas-phase NA photolysis. These results confirm that the dominant part of the OH-LIF signals during the liquid-phase experiment originated from OH radicals desorbed from the liquid NA surface.  Single-shot measurement. We also conducted single-shot experiments to reveal the origin of the photodesorbed OH radicals; i.e., whether they were formed directly from liquid NA molecules or from secondary photoproducts accumulated on the liquid surface. Figure 3 shows the time evolution of the OH-LIF signal intensities with respect to the 213 nm laser shots toward the liquid NA at the on-and off-resonance wavelengths for the excitation of OH radicals (Fig.   S4). The pulsed probe laser was set to be injected into the chamber during the single-shot experiments. At the on-resonance wavelength (black lines in Fig. 3), the intensities steeply  213 nm photolyses of gas-phase AA and liquid NA, because the absorption cross section and quantum yield of the OH formation have been well studied for gas-phase AA (see Table S1 for details) 10 (Table S1 gives details). 10,11 In addition, the photolysis of gas-phase AA was conducted in the crossed-beam configuration ( Fig. 1b) at 295 K at a partial pressure of 1.2 Pa for gas-phase AA in a total pressure of 80 Pa with nitrogen as a buffer gas. The number density of gas-phase AA ([AA]) was 2.9 × 10 14 cm −3 , and the photon fluence of the photolysis laser was 1.5 × 10 14 cm −2 . The relationship between the number density of the formed OH radicals and the OH-LIF signal intensity can be written as follows: where , , , and indicate the measured OH-LIF intensity (V s), a conversion factor between the OH-LIF intensity and the number density of OH radicals (V −1 s −1 cm −3 ), the efficiency of detecting OH radicals, and the solid angle of OH-LIF detection, respectively.
The present experiments had the electronics and optics arranged to make and common in the liquid-and gas-phase experiments. The detection efficiency, , depended on the diffusion of photodesorbed OH radicals and the decrease of their concentration by reactions with each other or with gas-phase AA or NA. In this study, the nitrogen buffer gas was introduced into the chamber up to 80 Pa, and hence the OH radicals during the gas-phase and liquid photolyses were cooled to room temperature due to repeated collisions with the buffer gas. The cooling of the OH radicals is necessary to derive the photoreaction cross section because this makes the diffusion velocities of the OH radicals identical in all the experiments, which enables us to quantitatively compare the photolyses of the gas-phase AA and the liquid NA. In addition, given the low number density of the formed OH radicals (~10 9 cm −3 ), OH-OH reactions can be ignored here.
Reactions between OH and gas-phase AA or NA can also be ignored for the following reason.
The rate constants for the reactions of OH radicals with organic compounds are typically 10 −13 -10 −10 cm 3 molecule −1 s −1 , 14 and the formed OH radicals are considered to be cooled to room temperature by multiple collisions with the nitrogen buffer gas in this study. Overall, the mean free paths of the OH radicals in the gas-phase AA or NA can be roughly estimated to be at least dozens of millimeters, which is much greater than the typical flight lengths of the OH radicals before LIF detection.
For valid comparison of the OH-LIF signal intensity of the gas-phase AA photolysis with that of the liquid-phase NA photolysis, should be the same in both experiments. Hence, the OH-LIF signal intensities measured at z = 0 mm and d′ = 0 mm allow the diffusion of the OH radical products to be neglected. However, it is impossible to measure the intensity at z = 0 mm due to the finite beam size of the probe laser. Therefore, we investigated the dependence of OH-LIF intensities during liquid NA photolysis on the detection position z (Fig. 4). The OH-LIF signal intensity became almost constant when the distance from the liquid surface to the probe laser (z) was less than 3 mm. This was not due to saturation of the detector, because the raw OH-LIF intensity measured during the gas-phase AA photolysis, before normalization by the photon fluence of the photolysis laser, was approximately two or three times larger than that measured during the liquid-phase NA photolysis (Table S1). Thus, Fig. 4 indicates that the diffusion of the formed OH radicals was sufficiently suppressed when z ≤ 3 mm. Assuming that the OH-LIF signal intensity measured at z = 1.5 mm was close to that for z = 0 mm, the present analysis compares the results of the liquid-phase photolysis of NA at z = 1.5 mm with those of the gasphase photolysis of AA at d′ = 0 mm. Both were measured at the laser delay time of 10 μs.   (Fig. S1 and ref. 8). In addition, OH radicals formed at micrometer depths would not desorb, owing to their small mean free path in the bulk. 16 As we cannot determine accurately the region in which the detected OH radicals formed, this study assumes that the OH photoproducts are desorbed only from the first layer of liquid NA. This  Table S1 for details of the parameters).
Consequently, the ratio of the photoreaction cross section of liquid NA to that of gaseous monomer AA is derived to be 1. . Therefore, it is concluded that the liquid-phase photolysis of NA forms fewer OH radicals than the gas-phase photolysis of AA. Table S1 gives the values and errors of each parameter used in this study.
A possible reason for this small OH radical yield is the formation of cyclic dimer structures at the liquid NA surface. Tyrode et al. used VSFG spectroscopy to find that AA molecules at the surface of an aqueous AA solution form cyclic AA dimers at high AA concentrations. 19 Molecules at the surface of neat liquid NA can also form cyclic NA dimers through strong hydrogen bonding, which might inhibit OH radical formation during the photolysis of liquid NA.

Sum-frequency generation (SFG) spectroscopy.
We used HD-VSFG spectroscopy to examine the possibility of cyclic dimer formation at the interface of air and neat NA. [20][21][22] An SFG signal arises only in media with no centrosymmetry such as interfaces, because SFG is a second-order nonlinear optical process. Therefore, VSFG spectroscopy can provide selective information on molecular orientations at the interface. Unlike infrared (IR) or Raman spectroscopy, HD-VSFG spectroscopy can distinguish the "up" or "down" orientations of a specific substituent group at the interface. 23-25 Figure 5A shows the SFG spectrum in the C=O stretching region; the plot of Im  (2) is the imaginary part of  (2) at the air-NA interface. The imaginary part, where a vibrational resonance appears as an absorptive line, is easily compared with bulk IR and Raman spectra. No vibrational band was observed from the neat NA liquid surface, and a vibrational band was clearly observed at 1714 cm −1 from the interface of air and a 1.0 mM aqueous NA solution ( Figure S5). There are two possible interpretations for the absence of C=O bands in Fig. 5A. One is a random orientation of NA molecules at the interface, as the SFG signal is generated when molecules have a specific orientation at the interface. The other possible interpretation is that a pair of NA molecules form a cyclic dimer structure when their carboxyl groups are hydrogen bonded facing each other. In this case, C=O stretching modes should be SFG inactive, because the hydrogenbonded carboxylic acid portion is locally centrosymmetric. We observed SFG spectra in the CH stretching region for the interface between air and neat NA to evaluate which of the two interpretations is more plausible. Figure 5B shows the Im  (2) spectrum in the CH stretching region for the interface between air and neat NA. In contrast to the C=O stretching region, vibrational bands appear clearly. The signs of the CH stretching bands indicate that the NA molecules were oriented with their terminal methyl groups toward the air at the interface. 26 This suggests that the absence of C=O bands in the spectrum of the air-neat NA interface was not due to NA molecules being disoriented, but instead due to the formation of cyclic dimers at the interface.
By comparing the signal intensities of the C=O stretching band between the SFG spectra of the air-1.0 mM aqueous NA and air-neat NA interfaces ( Fig. S5 and 5A), an occupancy of NA molecules without forming cyclic dimers (non-dimers) at the air-neat NA interface can be roughly estimated. In this study, the detection limit of the C=O stretching band signal is approximately one-fifth of that obtained from the air-1.0 mM NA aqueous interface (Fig. S5).
Assuming that all the NA molecules at the interface between air and 1.0 mM aqueous NA are monomers (non-dimers), the non-detection of the C=O stretching band in Fig. 5(A) suggests that the occupancy of non-dimers is at most 20% at the air-neat NA interface and the majority of NA molecules forms a cyclic dimer-structure, implying a high surface homogeneity of neat liquid NA.  Table S2 lists the obtained fitting parameters.
In this study, we developed new apparatus with the aim of quantitatively studying OH radical desorption during liquid organic photolysis. We succeeded in the direct detection of OH radicals formed by the 213 nm photolysis of neat liquid NA by using a highly sensitive LIF technique.
Comparing the results obtained during the photolysis of liquid-phase NA and gas-phase AA, we estimated the photoreaction cross section of OH radical desorption in the 213 nm photolysis of liquid NA to be 9.0(4.1) × 10 −22 cm 2 as an upper limit, which is only 1.2 ± 0.8 percent of that for the gas-phase AA monomer. This small value might have been due to the formation of cyclic NA dimers at the liquid surface, as indicated by HD-VSFG spectroscopy of the interface between air and neat NA. Moreover, some OH products may be consumed by reactions with surrounding NA molecules without escaping from the liquid surface, which can reduce the cross section for the OH radical desorption.
The photolysis of aqueous NA, namely the photolysis of hydrated NA molecules at the airwater interface, can play an important role in the troposphere. 7 By dividing the present vacuum chamber into photolysis and radical probing sections and connecting them with a differential pumping system, the photolysis of organic molecules on water at atmospheric pressure may be investigated. The photoreactions of NA molecules in the troposphere are considered to be initiated by a spin-forbidden singlet (S0)→triplet (T1) excitation with an absorption peak at around 270 nm. 2 This might cause the cross section of OH radical formation to differ significantly from that in the present 213 nm study. In addition, different photochemical processes such as fluorescence can also occur. Further study is currently in progress.

EXPERIMENTAL METHODS
LIF measurements. We developed a new experimental apparatus for the UV laser photolysis of liquid organic molecules and the detection of OH radicals by LIF (Fig. S2). To detect highly reactive OH radicals using LIF, an experimental chamber was evacuated to a background pressure of 1. or wetted wheels. 27,28 As shown in Fig. 1a, a 213 nm Nd:YAG pulsed laser (Lotis TII, LS-2137U-N and VM-TIM, YHG-5) was used for photolysis. This laser was P-polarized and incident on the liquid NA at the angle of 45° to its surface normal. The pulse energy and diameter of the beam were approximately 500 J and 4 mm, respectively, which allowed only single-photon absorptio.
Excited OH (A 2 ) was produced by unpolarized pulsed 282 nm radiation from a frequency- SFG spectroscopy. SFG spectra were measured using a laboratory-built HD-VSFG spectrometer, which has been described elsewhere. [20][21][22] In brief, a femtosecond regenerative amplifier (Coherent, Legend Elite; repetition rate, 1 kHz; center wavelength, 800 nm; pulse width, ~100 fs; average power, 3.0 W) was used to pump two optical parametric amplifiers. The fundamental output from the regenerative amplifier was divided into two. One third of the output was introduced into a white-light-seeded optical parametric amplifier (Coherent, TOPAS-800-fs) to obtain a broadband IR pulse at around 5800 nm (1700 cm −1 ) or 3450 nm (2900 cm −1 ). The remaining two thirds of the output was frequency-doubled to 400 nm by a narrow band second harmonic generator (Coherent, SHBC). The spectrally narrowed second harmonic pulse (FWHM: 8 cm −1 ) was used to pump a white-light-seeded optical parametric amplifier (TOPAS-400-ps-WL) to obtain a visible pulse at 596 nm. The visible and IR pulses were overlapped in a y-cut quartz crystal (10 m thick) to generate a broadband sum-frequency (SF) pulse used as a local oscillator (LO). The transmitted visible, IR, and LO pulses were refocused by a concave mirror onto the sample surface or a z-cut quartz surface for reference. The incident angles of the visible and IR pulses were 72° and 60° from the surface normal, respectively. The LO pulse was passed through a 1.5 mm (CH stretching region) or 3 mm thick (C=O stretching region) fused silica plate located between the concave mirror and the sample surface to delay it relative to the visible and IR pulses by ~2.5 ps for the CH stretching region or ~5 ps for the C=O stretching region. The reflected LO and SF pulses from the sample surface were passed through a polarization analyzer to select the polarization of the SFG signal. They were introduced into a polychromator (Horiba Jovin Yvon, TRIAX550; focal length, 550 mm; grating, 1200 grooves/mm) through a prism monochromator (Jasco, CT25-UV), and interfered with each other in the frequency domain. The interfered pulses were detected by a liquid nitrogen-cooled charge coupled device detector (Roper Scientific, LN/CCD-1340/400-EB). The energy of the visible and IR pulses at the sample was ~6 and ~2 J/pulse, respectively. The polarization combination for the SF, visible, and IR pulses was SSP (S-polarized SF, S-polarized visible, and P-polarized IR pulses). The height of the sample surface was monitored by a displacement sensor (Keyence, LT8110) and maintained at the same height of the reference z-cut quartz surface within an accuracy of 5 m. The obtained SFG spectra were normalized with those of the reference z-cut quartz. For measurements of the C=O stretching region, the sample compartment was purged using dry air to suppress the extinction of the IR pulse by water vapor absorption.
Neat NA liquid and aqueous NA solution samples were placed in a Teflon trough (65 mm × 65 mm).

Supporting Information
The following files are available free of charge.
UV absorption spectrum of neat liquid NA (Fig. S1); photographs of the experimental setup (