Product state distribution rotational

The H2O molecules are cooled in a supersonic expansion to a rotational temperature of 10K before photodissociation. The evidence for pathway competition is an odd-even intensity alteration in the OH product state distribution for rotational quantum numbers V = 33 45. This intensity alternation is attributed to quantum mechanical interference due to the N-dependent phase shifts that arise as the population passes through the two different conical intersections. 

Figure 12. Potential energy contour plots for He + I Cl(B,v = 3) and the corresponding probability densities for the n = 0, 2, and 4 intermolecular vibrational levels, (a), (c), and (e) plotted as a function of intermolecular angle, 0 and distance, R. Modified with permission from Ref. 40. The I Cl(B,v = 2/) rotational product state distributions measured following excitation to n = 0, 2, and 4 within the He + I Cl(B,v = 3) potential are plotted as black squares in (b), (d), and (f), respectively. The populations are normalized so that their sum is unity. The l Cl(B,v = 2/) rotational product state distributions calculated by Gray and Wozny [101] for the vibrational predissociation of He I Cl(B,v = 3,n = 0,/ = 0) complexes are shown as open circles in panel (b). Modified with permission from Ref. [51].

Gray and Wozny [101, 102] later disclosed the role of quantum interference in the vibrational predissociation of He Cl2(B, v, n = 0) and Ne Cl2(B, v, = 0) using three-dimensional wave packet calculations. Their results revealed that the high / tail for the VP product distribution of Ne Cl2(B, v ) was consistent with the final-state interactions during predissociation of the complex, while the node at in the He Cl2(B, v )Av = — 1 rotational distribution could only be accounted for through interference effects. They also implemented this model in calculations of the VP from the T-shaped He I C1(B, v = 3, n = 0) intermolecular level forming He+ I C1(B, v = 2) products [101]. The calculated I C1(B, v = 2,/) product state distribution remarkably resembles the distribution obtained by our group, open circles in Fig. 12(b). 

The probability distribution for the n = 2 intermolecular level. Fig. 12c, indicates that this state resembles a bending level of the T-shaped complex with two nodes in the angular coordinate and maximum probability near the linear He I—Cl and He Cl—I ends of the molecule [40]. The measured I C1(B, v = 2f) rotational product state distribution observed following preparation of the He I C1(B, v = 3, m = 2, / = 1) state is plotted in Fig. 12d. The distribution is distinctly bimodal and extends out to the rotational state, / = 21,... 

Photodissociation dynamics [89,90] is one of the most active fields of current research into chemical physics. As well as the scalar attributes of product state distributions, vector correlations between the dissociating parent molecule and its photofragments are now being explored [91-93]. The majority of studies have used one or more visible or ultraviolet photons to excite the molecule to a dissociative electronically excited state, and following dissociation the vibrational, rotational, translational, and fine-structure distributions of the fragments have been measured using a variety of pump-probe laser-based detection techniques (for recent examples see references 94-100). Vibrationally mediated photodissociation, in which one photon... 

The quantum product state distributions from the reaction show a similar dichotomy for EC<1 kcal/mol and EC>1 kcal/mol. Focusing on the rotational state distribution for the dominant HF(tf = 2) product, in Figure 3.5 we show the ICS for F+HD HF(v = 2,/ ) as a function off and Ec. The scattering calculations show a clear change in the rotational product distribution between low- and high-energy scatterings. The rotational distribution at low... 

An optical parametric oscillator (OPO) was used to select HF(n = 1) in each of the rotational states J = 0—7 in a study of the effect of rotational excitation on the product state distributions in the reaction ... 

In this chapter we elucidate the state-specific perspective of unimolec-ular decomposition of real polyatomic molecules. We will emphasize the quantum mechanical approach and the interpretation of the results of state-of-the-art experiments and calculations in terms of the quantum dynamics of the dissociating molecule. The basis of our discussion is the resonance formulation of unimolecular decay (Sect. 2). Summaries of experimental and numerical methods appropriate for investigating resonances and their decay are the subjects of Sects. 3 and 4, respectively. Sections 5 and 6 are the main parts of the chapter here, the dissociation rates for several prototype systems are contrasted. In Sect. 5 we shall discuss the mode-specific dissociation of HCO and HOCl, while Sect. 6 concentrates on statistical state-specific dissociation represented by D2CO and NO2. Vibrational and rotational product state distributions and the information they carry about the fragmentation step will be discussed in Sect. 7. Our description would be incomplete without alluding to the dissociation dynamics of larger molecules. For them, the only available dynamical method is the use of classical trajectories (Sect. 8). The conclusions and outlook are summarized in Sect. 9. 

In this section, we shall focus exclusively on the scalar properties of the fragments and consider the vibrational and rotational product state distributions (PSD s) following the dissociations of HCO, NO2, and H2CO discussed in Sects. 5 and 6. An in-depth introduction to the vast and fascinating field of product state analysis can be found in Ref. 20 (Chapters 9, 10, and 11). Recently, the PSD s of several representative groups of molecules were reviewed in Ref. 306. 

In product state spectra, the pump laser wavelength is fixed and the probe laser is scanned to obtain product rovibronic spectra. Analyses show how the product state distributions depend on the portion of the potential surface initially excited. Rotational state distributions of IlgHfX Z, V, N) were obtained using the A n,/2 <- X Z system. 

Internal energy partitioning between vibration and rotation is very different for Ai and A2 symmetries 18% of the internal energy goes into rotation for the Ai symmetry, in contrast with 50% for the A2 symmetry. This reflects itself in the product state distributions of Fig. 11, which have a maximum for low rotational quantum number j in the Ai symmetry, but for j near 15 for the A2 case. 

Finally, what is the role of IVR in photodissociation Very recently, we monitored the photodissociation of t-stilbene-He(Ar) complexes in real time (see Fig. 53)71 using polarized (coherent) excitation and analyzed product state distribution. The surprising finding was that the dissociation process, occurring subsequent to IVR, leaves bare t-stilbene in coherent rotational states (see Fig. 54 and compare the results with those in Fig. 47 for bare t-stilbene). The initial coherence induced in the... 

In the experimental studies of state specific NO2 unimolecular dissociation (Miy-awaki et al., 1993 Hunter et al., 1993 Reid et al., 1994, 1993), NO2 is first vibra-tionally/rotationally cooled to 1 K by supersonic jet expansion. Ultraviolet excitation is then used to excite a NO2 resonance state which is an admixture of the optically active and the ground electronic states. [It should be noted that in the subpicosecond experiments by Ionov et al. (1993a) discussed in section 6.2.3.1, a superposition of resonance states is prepared instead of a single resonance state.] The NO product states are detected by laser-induced fluorescence. Both lifetime and product energy distributions for individual resonances are measured in these experiments. A stepwise increase in the unimolecular rate constant is observed when a new product channel opens. Fluctuations in the product state distributions, depending on the resonance state excited, are observed. The origin of the dynamical results is not clearly understood, but it apparently does not arise from mode specificity, since analyses of... 

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