The assignment of the Rydberg transitions in the electronic absorption spectrum of formaldehyde

Abstract

The electronic spectra of H 2 CO, D 2 CO, HDCO, and D2 \3CO have been recorded between 1250 and 2000 A and the vibrational fine structure associated with the n-> 3 s, 3 p" 3 py, 3 p" 3 d transitions assigned. The electronic origin bands in the Rydberg transitions show quite unusual isotope effects, with the v(D 2 CO)-v(H 2 CO) shift for the n-> 3 d transition being somewhat smaller than the limiting value of the ion while the lower energy n->3s and 3py transitions have anomalously large values. As these effects cannot be accommodated by the changes in the totally symmetric in-plane frequencies on excitation, they are attributed to large differences in Vs and V6 in the upper and lower electronic states. Direct information concerning the reduction in V6 on excitation comes from the identification of a series of low frequency bands in the n-> 3 Py system of D 2 CO which were assigned to quanta in V6' The potential function which was derived from this data was found to be very anharmonic and could contain a double minimum. The reduction in V( Q,;) at large Q,; was attributed to a coupling between the Rydberg IA1(n, 3py) state and a second IB2 state which lies at higher frequencies. An estimate of the coupling between these states lead to a value of 1480 em ~ I for the vibronic matrix element. While this value is entirely compatible with what would be expected for valence-valence coupling, it is somewhat larger than what would have been anticipated for vibronic coupling between Rydberg states. RESULTS Electronic structureThe electronic structure of formaldehyde in its ground and higher electronic states may be derived from molecular orbital theory as a linear combination

Photo-Absorption Studies on Formaldehyde Using Synchrotron Radiation at Indus I

“…A number of weak vibrational structures at 8.593 eV with energy separation of 170-411 cm À1 correspond to valence transition p À p à . Irregular long progressions observed here indicate a significant distortion from the ground state and also strong interaction of the n-3d and n-3p Rydberg transitions [12,15] with neighboring valence states.…”

“…[7] The electronic spectrum of HCHO has been investigated extensively since the 1930s, focusing on its Rydberg and intravalence transitions. [8][9][10][11][12][13][14][15][16][17][18][19][20][21] The electronic spectrum in the vacuum-ultraviolet (VUV) region ($7-11 eV) was recorded by both electron impact and optical techniques. [16,17] Further the photoabsorption spectra that were measured using dipole (e,e) spectroscopy in the photon energy region 3-200 eV were used to calculate absolute oscillator strengths by method of Cooper et al [19] In spite of the size and simplicity of formaldehyde, assignments of the Rydberg and valence states were found difficult because of strong interactions between them.…”

Electronic excitations from a linear-response range-separated hybrid scheme

“…We calculate vertical excitation energies and oscillator strengths of five small molecules, N 2 , CO, H 2 CO, C 2 H 4 , and C 6 H 6 , which have already been extensively studied theoretically [13,27,[56][57][58][59] and experimentally [60][61][62][63]. In order to have unique, comparable references, equationof-motion coupled-cluster singles doubles (EOM-CCSD) calculations were done in the same basis with the quantum chemistry program Gaussian 09 [64].…”

The electronic structure of the H2CO+ radical and higher Rydberg states of H2CO

“…According to Guyon et al [8], the p ,3p photoionization spectrum contains a short vibrational progression with a rather small spacing (< 300 cm -1 ), which was assumed to correspond to t 4 , since a similar feature had previously been reported for 1 1 B 2 (n ,3s ) of H 2 CO [30,31,43]. However, in a more detailed study of this H 2 CO band, Lessard and Moule [44] came to the conclusion that the complex vibrational structure involved excitations of the b 2 modes as well. As we showed in [30,31], the anomalous vibrational structure of 1 B 2 (n ,3s ) of H 2 CO results from its vibronic interaction with 1 B 1 (s ,p * ), a process requiring a C 1 intermediate structure essentially involving all vibrational modes.…”