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.