Pure vibrational spectroscopy of S0 formaldehyde by dispersed fluorescence

Abstract

Dispersed fluorescence spectra from the 000 rotational level of 40, 41, 51, and 3141 S1 formaldehyde (H2CO) have been recorded. From these spectra, 198 new vibrational states have been assigned with energies up to 12 500 cm−1, and their positions have been determined to within an uncertainty of 1 cm−1. The assignment of vibrational lines to specific vibrational states becomes increasingly difficult at the higher energy regions of the spectra (≳9000 cm−1) due to extensive state mixing. Harmonic and first-order anharmonic vibrational constants were extracted from fits to these vibrational states. For states with highest zero-order coefficient squared greater than 35%, the standard deviation of the spectroscopic fit is 6.9 cm−1. For states which are lower energy (<9500 cm−1) and relatively pure (zero-order coefficient squared greater than 0.75 or largest in a given normal mode combination), the standard deviation is 1.7 cm−1. Good agreement with ab initio vibrational constants calculated by Martin et al. [J. Mol. Spectrosc. 160, 105 (1993)] is achieved, except in cases where all observed states contributing to the determination of a particular constant are significantly mixed. These deviations are readily explained by a consideration of anharmonic vibrational interactions that occur among specific combinations of normal modes. The average mean deviation between all experimentally determined energies and a recent theoretical calculation by Burleigh et al. [J. Chem. Phys. 104, 480 (1996)] is 2.6 cm−1.

Rotational analysis of the 2ν5 band of formaldehyde

“…We have been able to assign levels up to J′ = 20 and K a ' = 5 and have identified several perturbations. As information with moderate accuracy (~ 1 cm -1 ) on all the states in the region is available from previous work, including an experimental study by Bowens et al [29] and theoretical study by Burleigh et al [30], we were able to assign a source to all the perturbations. The model we developed for the perturbations enabled us to fit the spectrum with an average error of 0.013 cm -1 , compared to a line width of the experimental spectrum of 0.022 cm The spectrum used for the analysis is described in [24]; it was of a 0.02 Torr sample of HCHO and covered the range 5600 -5730 cm -1 .…”

“…Fortunately, there is significant information on the other vibrational states at similar energies to allow us to identify the states responsible for the perturbations. The bulk of this information comes from dispersed fluorescence spectroscopy work via the 0 00 rotational level of the zero point level, ν 4 , ν 5, and ν 3 +ν 4 vibrational states of the S 1 state of formaldehyde, with the measured states and band origins given in ref [29]. Table 1 shows the available information on the states in the region of 2ν 5 in the energy range of 5540 cm -1 to 6100 cm -1…”

The rotationally-resolved absorption spectrum of formaldehyde from 6547 to 6804cm−1

“…We have measured the formaldehyde spectrum from 6547 to 6804 cm Table Table 2 Combination bands between 6500 and 6830 cm −1 from the dispersed fluorescence spectra of Bouwens et al [13]. Tentatively identified combination bands observed in our spectrum are indicated with asterisks.…”

“…The H 2 CO molecule has C 2v symmetry and six normal modes (Table 1), with several combination bands between 6500 and 6830 cm −1 (Table 2). These transitions have been observed with a 1 cm −1 resolution by Bouwens et al in a jet experiment using dispersed fluorescence spectroscopy from a number of vibrational bands in the S 1 excited state [13]. The H 2 CO combination bands consist of three to five normal modes.…”

Introdução aos Métodos Algébricos Unidimensionais em Espectroscopia Molecular: Sistemas Diatômicos

“…A restrição a moléculas planares com C 2v será por comodidade, embora isto não signifique que estes sistemas apresentem espectros vibracionais simples. Em particular, estaremos interessados nas moléculas tri-atômicas [44,45,46] H 2 O, SO 2 , NO 2 (simétrica), etc., e na molécula tetra-atômica H 2 CO e seus derivados [47].…”