Photodissociation dynamics of the reaction H{sub 2}CO{yields}H+HCO via the singlet (S{sub 0}) and triplet (T{sub 1}) surfaces
- School of Chemistry, University of Sydney, New South Wales 2006 (Australia)
We have explored the photodissociation dynamics of the reaction H{sub 2}CO+h{nu}{yields}H+HCO in the range of 810-2600 cm{sup -1} above the reaction threshold. Supersonically cooled formaldehyde was excited into selected J{sub Ka,Kc} rotational states of six vibrational levels (1{sup 1}4{sup 1}, 5{sup 1}, 2{sup 2}6{sup 1}, 2{sup 2}4{sup 3}, 2{sup 3}4{sup 1}, and 2{sup 4}4{sup 1}) in the A(tilde sign)({sup 1}A{sub 2}) state. The laser induced fluorescence spectra of the nascent HCO fragment provided detailed product state distributions. When formaldehyde was excited into the low-lying levels 1{sup 1}4{sup 1}, 5{sup 1}, and 2{sup 2}6{sup 1}, at E{sub avail}<1120 cm{sup -1}, the product state distribution can be modeled qualitatively by phase space theory. These dynamics are interpreted as arising from a reaction path on the barrierless S{sub 0} surface. When the initial states 2{sup 2}4{sup 3} and 2{sup 3}4{sup 1} were excited (E{sub avail}=1120-1500 cm{sup -1}), a second type of product state distribution appeared. This second distribution peaked sharply at low N, K{sub a} and was severely truncated in comparison with those obtained from the lower lying states. At the even higher energy of 2{sup 4}4{sup 1} (E{sub avail}{approx_equal}2600 cm{sup -1}) the sharply peaked distribution appears to be dominant. We attribute this change in dynamics to the opening up of the triplet channel to produce HCO. The theoretical height of the barrier on the T{sub 1} surface lies between 1700 and 2100 cm{sup -1} and so we consider the triplet reaction to proceed via tunneling at the intermediate energies and proceed over the barrier at the higher energies. Considerable population was observed in the excited (0,0,1) state for all initial H{sub 2}CO states that lie above the appearance energy. Rotational populations in the (0,0,1) state dropped more rapidly with (N,K{sub a}) than did the equivalent populations in (0,0,0). This indicates that, although individual rotational states are highly populated in (0,0,1), the total v{sub 3}=1 population might not be so large. Specific population was also measured in the almost isoenergetic K{sub c} and J states. No consistent population preference was found for either asymmetry or spin-rotation component.
- OSTI ID:
- 20991304
- Journal Information:
- Journal of Chemical Physics, Vol. 127, Issue 6; Other Information: DOI: 10.1063/1.2752161; (c) 2007 American Institute of Physics; Country of input: International Atomic Energy Agency (IAEA); ISSN 0021-9606
- Country of Publication:
- United States
- Language:
- English
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