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Science 10 March 2006:
Vol. 311. no. 5766, pp. 1443 - 1446
DOI: 10.1126/science.1123397
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Reports

Signatures of H2CO Photodissociation from Two Electronic States

H. M. Yin1, S. H. Kable1*, X. Zhang2 and J. M. Bowman2*

1 School of Chemistry, University of Sydney, Sydney, NSW 2006, Australia.
2 Department of Chemistry and Cherry L. Emerson Center for Scientific Computation, Emory University, Atlanta, GA 30322, USA.


Figure 1 Fig. 1. Relevant potential energy slices of H2CO producing the radical H + HCO products. Energies are based on ab initio calculations and do not include zero-point energy (6). [View Larger Version of this Image (19K GIF file)]
 

Figure 2 Fig. 2. Laser-induced fluorescence (LIF, left) and photofragment excitation (Phofex, right) spectra of H2CO. Region A: H2CO is bound and hence no structure is seen in the phofex spectrum. Region B: H2CO is unbound on the S0 surface but bound on the T1 surface. Region C: H2CO is within the uncertainty limits of the T1 barrier height. Region D: H2CO is unbound on both S0 and T1 surfaces. The state labels refer to excitation of the vibrational modes of H2CO in the S1 state, e.g., 2241 indicates two vibrational quanta in the CO stretch (mode 2) plus one quantum of excitation in a bending vibration (mode 4). [View Larger Version of this Image (24K GIF file)]
 

Figure 3 Fig. 3. Rotational state distributions of the HCO fragment following dissociation of H2CO. Regions B to D correspond to those in Fig. 2. The distributions reflect dynamics for HCO production from the S0 and T1 states that are completely different. The distributions in region B are well modeled by phase space theory, whereas those in region D are in excellent agreement with QCT trajectory calculations on an ab initio T1 surface in this work. The data in this figure cover ~2600 cm–1 of excess energy and show the distribution over the N rotational quantum number of HCO for Ka = 0 and v = 0. [View Larger Version of this Image (24K GIF file)]
 

Figure 4 Fig. 4. Comparison between experimental and QCT results for two initial states of H2CO (2441 and 112141), which should lie above the triplet barrier. The left column shows the HCO population distribution over the N rotational states for Ka = 0 and v = 0. The middle column shows the same distribution for the (0,0,1) vibrational state. The right column shows the distribution over the Ka states (integrated over N) for v = 0. The experimental data were not available for the 112141 state due to the low signal. The close agreement indicates that the T1 surface dominates the reaction for these specific levels. [View Larger Version of this Image (30K GIF file)]
 

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