Dissociative excitation of HCOOH by single-vacuum ultraviolet and two-ultraviolet photon

Abstract

Dissociative excitation processes of HCOOH in the vacuum ultraviolet (VUV) region were studied by single-VUV photon with synchrotron radiation source and by two-ultraviolet (UV) photon with KrF excimer laser. In the VUV dissociation, fluorescence excitation cross sections for the OH(A) and HCOO* were separately determined in the 106–155 nm region. The branching fraction was found to be a function of the VUV excitation wavelength. The magnitude is σOH(A)/[σOH(A)+σHCOO*]=0.13 at 124.5 nm and gradually increases to 0.39 at 110 nm. In the UV multiphoton dissociation at 249 nm, OH(A) and HCOO* fragments were also identified by a fluorescence spectrum. The production of OH(A) was shown to take place in the two-UV photon absorption of HCOOH. Nascent rotational and vibrational (V/R) state distributions of OH(A 2Σ+) produced via the photodissociation at a single excitation energy of 9.96 eV (124.5×1/249 nm×2), HCOOH+nhν(n=1,2)→HCO+OH(A 2Σ+), were determined by simulation analysis of the dispersed fluorescence spectra. The internal state distributions were found to be of the relaxed type, and rotational distribution could be approximated by a Boltzmann distribution. One-VUV photon excitation gave the best-fit rotational temperature Tr(v′=0)=3000 K and vibrational population ratio Nv′=1/Nv′=0=0.14, while two-UV photon excitation showed Tr(v′=0)=2000 K with Nv′=1/Nv′=0=0.12. Possible mechanisms for the OH(A) formation by both excitation sources were examined based on simple theoretical models. The degree of internal excitation is not consistent with a direct dissociation on a repulsive surface, and neither is a dissociation from a long-lived intermediate state. The formation of OH(A 2Σ+) is interpreted as dissociation of an electronically excited intermediate state, leading to the formation of OH(A)+CHO, populated competitively via an electronic predissociation process. The substantially different V/R distributions observed are dependent on the excited precursor state initially accessed, and may result from the constraint in the competing predissociation step that follows.

Photophysical studies of formic acid in the VUV. Absorption spectrum in the 6–22 eV region

“…Earlier absorption and fluorescence spectra with comparable resolution were limited to measurements below 11.5 eV. 13,14 Our photoelectron spectra 11 were measured at a higher resolution than in previous studies. [15][16][17] No previous measurements of the ionization yields of formic acid have been reported.…”

“…1). Suto et al 13 published absorption spectra over the 4.96-11.7 eV region, and Tabayashi et al 14 in the 8-11.7 eV region, using synchrotron radiation as the spectral source. Our cross-sections for sharp bands are 8-12% higher than those of Suto et al, but are somewhat lower than those of Tabayashi et al, whose measurements have an uncertainty of AE15%.…”

Destruction of formic acid by soft X-rays in star-forming regions

“…The photodissociation of formic acid has been studied experimentally and theoretically in the vacuum ultraviolet (VUV) region (Su et al 2000;Tabayashi et al 1999;Schwell et al 2002). However the results cannot explain the ice/gas ratio, since about 20% of formic acid survives the UV radiation.…”

Dissociative excitation of acetyl cyanide by ultraviolet multiphoton absorption

“…17 Multiphoton excitation of acetyl cyanide was performed with 0.4 -4 mJ pulses of 292.0 nm laser light. A dye laser ͑Lambda Physik, SCANmate2E͒, pumped by the second har-monic output of a Nd:YAG laser ͑Quanta-Ray, INDI-40-10͒ at a repetition rate of 10 Hz, was driven at 584.0 nm with Rhodamine B.…”