The photodissociation process

The photodissociation process takes place most frequently at excitation of the molecule to a non-bonded state, with subsequent dissociation into products. Since the angular part of the transition probability, according to Chapter 2, is still dependent on the mutual orientation of the E-vector of the initiating light beam and on the transition dipole moment d, one may expect spatial anisotropy of angular momenta distribution both in the dissociation products and in the set of molecules which remains undestroyed. 

Let us start with the second effect which is most close to the ideas discussed in Chapter 3 in connection with optical polarization by de- 

If photodissociation takes place, as is frequently the case, under the action of a laser pulse, short with respect to relaxation time, then the ratio AN/N for the remaining undecayed molecules in the JM) state directly after the pulse is simply 

The idea and the theoretical description of creating alignment in neutral molecules which remained undestroyed after photodissociation may be found in the works by Bersohn and Lin [68], Zare [400] and Ling and Wilson [272], The experimental proof of alignment of non-decayed molecules, as a rule, was of an indirect nature. 

A scheme elucidating the interconnection between the linearly polarized light vector E which determines the laboratory 2-axis, and d, v/ is given in Fig. 6.1 for a parallel transition type in a parent diatomic molecule AB (see Fig. 1.5(a, c)), where r d leads to v/ d. Using (2.1) we obtain the desired probability W (6f) of angular distribution of photofragments (Fig. 6.1 shows only one of them on the right) as 

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A. Garcia-Vela, R.B. Gerber, and D.G. Imre. Mixed quantum wave packet/classical trajectory treatment of the photodissociation process ArHCl - Ar-I-H-I-Cl. J. Chem. Phys., 97 7242-7250, 1992. 

In the case of NO2, for each photon absorbed below 400 nm, photodissodation occurs. For other photoabsorbers, HNOj and aldehydes, the photodissociation process leads to the formation of free radicals. 

Recently, the photodissociation process, HOD + hv — OD + H, has also been studied at the 121.6 nm using the experimental technique described above. Contributions from H2O were then subtracted from the results of the mixed sample. The experimental TOF spectra of the H atom from HOD were then converted into translational energy spectra in the center-of-mass frame. Figure 17 shows the translational energy spectra of the H-atom products at 121.6 nm excitation using two different polarization schemes... 

The photodissociation processes, occurring with the benzylamine molecule and the corresponding spectral thresholds, together with the relative abundance of the fragment ions are given in Table I,... 

By using the presented second-generation gas electron diffraction apparatus, it would also be possible to probe vibrational motion in real time. Especially when a molecule is photodissociated, a series of snapshots of a diffraction pattern would facilitate understanding the photodissociation process because it describes how a molecule vibrates in the course of the separation of two frag-... 

The total energy in the photolysis stage, denoted ab> is ab = Eg + ho), where co is the frequency of the photolysis laser. Hence, following the photodissociation process, fragment B is described by... 

Photochemistry of ammonia has been reviewed recently by McNcsby and Okabe (684). The photodissociation process of ammonia appears to follow the spin conservation rules [Okabe and Lenzi (762)] in that the spin-allowed process... 

In the pulsed molecular beam studies, the results were used to calculate an impact parameter for the photodissociation process. From these impact parameters it was concluded that the recoiling CN fragment did not take the lowest energy path when it was departing from the halogen atom. Rather, because of strong impulsive motion, the trajectory that the CN rsdical... 

Guest et al. have studied the polarization of the fluorescence that results from the B( E+) when C1CN is photolyzed at 157.6 nm with a F2 laser (158). They find that the absorption dipole is parallel to the C1CN internuclear axis and that the photodissociation process is direct. 

An interesting approach has been attempted in the works of Vasyutin-skii and coworkers [252, 254, 380] who showed, for the case of Csl, TIBr and Rbl, that conservation of angular momentum in the photodissociation process caused by circularly polarized light produces orientation of the atoms formed, the latter enabling measurement by means of magnetic resonance of the atomic products created. This is so if the lifetime of the... 

Iii turn, as the molecular fragments separate in the photodissociation process, these states go over to the noninteracting radiatively decoupled states,... 

Finally, we make a few additional remarks. First, note that a pure number state is a3j= state whose phase 0k is evenly distributed between 0 and 2n. This is a consequence of the commutation relation [3] between Nk and e,0 <. Nevertheless, dipole mafKi w elements calculated between number states are (as all quantum mechanical amplitudes) well-defined complex numbers, and as such they have well-defined phajje j S Thus, the phases of the dipole matrix elements in conjunction with the mode ph f i f/)k [Eq. (12.15)] yield well-defined matter + radiation phases that determine the outcome of the photodissociation process. As in the weak-field domain, if only gJ one incident radiation mode exists then the phase cancels out in the rate expres4<3 [Eq. (12.35)], provided that the RWA [Eqs. (12.44) and (12.45)] is adoptedf However, in complete analogy with the treatment of weak-field control, if we irradh ate the material system with two or more radiation modes then the relative pb between them may have a pronounced effect on the fully interacting state, phase control is possible. 

As mentioned above, the interpretation of the asymmetry of a a E)/E for a triatomic molecule is much more difficult than for a diatomic molecule. However, the asymmetry is a relevant phenomenological parameter which characterizes the XS in addition to the centre, the width and the amplitude. The isotopic (isotopologue) dependence of these four parameters is of special interest when dealing with the photodissociation processes [9,15]. In fact, this aspect was our initial motivation when starting this study and will be treated in the future. Below, we characterize the XS asymmetry of three important molecules O3, SO2 and CO2. 

Recently in our laboratory we have initiated a program to study the photoabsorption processes of metal vapors throughout the UV and EUV region. Our research interests are (1) to obtain the absolute cross section measurement of atomic and molecular metal vapors, (2) to study the photoionization processes of molecular metal species, and (3) to study the photodissociation processes of molecular metal ions. Several experimental methods such as the heat-pipe absorption spectroscopy, photoionization mass spectroscopy, and electron-ion coincidence technique, will be used in the study. This report summarizes our first experiment using heat-pipe absorption spectroscopy. 

The initial step (ultrafast CO loss within 50 fs) of the photodissociation process of CO-ligated iron porphyrin imidazole (Scheme 3), the model system for the CO-Ligated hemoglobin and myoglobin, has been investigated by means of TD-DFT. On the basis of the one-dimensional PEC... 

A few examples may demonstrate this. The approximation has been used to treat the photodissociation processes of NOHCl [33] and NO2 [34] which include three internal modes. Five dimensional time dependent calculations were performed on the photodissociation of CH3I [35, 36]. The state-to-state chemistry has been investigated for the reactive scattering of H-1-H2 (v=0,l) —> H2 (v=0,l) -1- H collinear system [37]. The MCTDSCF approach has also been applied to surface chemistry, in particular H2 dissociation on a transition metal surface [38] the photodissociation of CH3I on MgO surface [39, 40] and to inelastic molecule-surface scattering [41, 42]. Recently, the MCTDSCF method has been used to investigate multimode effects in the absorption spectrum of pyrazine taking into account 14 vibrational modes [43]. 

The S matrix elements, which occur in Eq. (7), contain information about the dynamics or scattering on the final or upper state electronic energy surface, but they do not contain information about the probability of the photodissociation process. 

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