Abstract
We have developed and tested two-photon excited fragment spectroscopy (TPEFS) for detecting HNO3 in pulsed laser photolysis kinetic experiments. Dispersed (220-330 nm) and time-dependent emission at (310 ± 5) nm following the 193 nm excitation of HNO3 in N2, air and He was recorded and analysed to characterise the OH(A2Σ) and NO(A2Σ+) electronic excited states involved. The limit of detection for HNO3 using TPEFS was ∼5 × 109 molecule cm-3 (at 60 torr N2 and 180 μs integration time). Detection of HNO3 using the emission at (310 ± 5 nm) was orders of magnitude more sensitive than detection of NO and NO2, especially in the presence of O2 which quenches NO(A2Σ+) more efficiently than OH(A2Σ). While H2O2 (and possibly HO2) could also be detected by 193 nm TPEFS, the relative sensitivity (compared to HNO3) was very low. The viability of real-time TPEFS detection of HNO3 using emission at (310 ± 5) nm was demonstrated by monitoring HNO3 formation in the reaction of OH + NO2 and deriving the rate coefficient, k2. The value of k2 obtained at 293 K and pressures of 50-200 torr is entirely consistent with that obtained by simultaneously measuring the OH decay and is in very good agreement with the most recent literature values.
Highlights
HNO3 is an important atmospheric trace gas and its ultra-violet photo-dissociation has been the subject of numerous studies.[1]The photo-dissociation of HNO3 can be divided into three channels, leading to formation of OH, O-atoms or H-atoms, the relative importance of which depends on the wavelength.HNO3 + hn (I) - OH(X) + NO2(X2A1) - OH(X) + NO2(12B2)HNO3 + hn (II) - O(1D) + HONO(X1A0) - O(3P) + HONO(a3A00)(R1a) (R1b) (R1c) (R1d)- O(3P) + HONO(X1A0) (R1e)HNO3 + hn (III) - H(2S) + NO3 (R1f)At wavelengths (l) greater than 250 nm, the n–p* transition in HNO3 leads to photodissociation into predominantly OH + NO2 (FI250nm 4 0.97) with a weak contribution from O-atom formation (FI2I50nm = 0.03).The formation of NO3 and H photo-fragments (Channel III)
The value of k2 obtained at 293 K and pressures of 50–200 torr is entirely consistent with that obtained by simultaneously measuring the OH decay and is in very good agreement with the most recent literature values
Our two-photon excited fragment spectroscopy (TPEFS) measurement of HNO3 monitors a fluorescence signal that is transmitted through an interference filter (310 Æ 5 nm) that biases detection to the strong OH (0,0) emission lines
Summary
HNO3 is an important atmospheric trace gas and its ultra-violet photo-dissociation has been the subject of numerous studies.[1]. Spin conservation and energy considerations the authors were able to demonstrate that OH(A) was not formed directly but via the photolysis of electronically excited HONO, probably in its metastable lower triplet state (a3A00) They used these findings to develop a new method (laser-photolysis fragmentfluorescence, LPFF) for the measurement of HNO3 in the atmosphere.[15,16] Recently, Winiberg et al.[17] reported results. We investigate the two-photon photodissociation of HNO3 at 193 nm and demonstrate the application of two-photon excited fragment spectroscopy (TPEFS) detection of HNO3 in real-time (flash photolysis) kinetic studies For the latter we re-measured the well-known rate coefficient[19,20] of the reaction between OH and NO2 (R2) by monitoring the HNO3 product by TPEFS and by near-simultaneous detection of OH via Laser Induced Fluorescence (LIF): OH + NO2 + M - HNO3 + M - HOONO + M (R2a) (R2b). As the application of TPEFS in kinetic studies of HNO3 will depend on its selectivity, we characterisd the sensitivity of TPEFS at 193 nm for detection of several other trace gases, including NO and NO2 which are often present (as impurities or products) in reaction systems involving HNO3
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