Excitation, overtone

Callegari A, Rebstein J, Muenter J S, Jost R and Rizzo T R 1999 The spectroscopy and intramolecular vibrational energy redistribution dynamics of HOCI in the u(OH) = 6 region, probed by infrared-visible double resonance overtone excitation J. Chem. Phys. 111 123-33... 

It is also possible that the unimolecular reaction takes place with the molecule in the electronic ground state, but it requires very intense fields to generate so-called multiphoton or direct overtone transitions that is, transitions from the vibrational ground state of the type 0 —> n, where n > 1. The opening of the cyclo-butene ring to form butadiene is an example of a unimolecular reaction induced by direct overtone excitation ... 

In a normal-mode picture, the C-H stretching of cyclo-butene is, essentially, a pure normal mode. There are two types of CH bonds, and the normal modes involve either olefinic CH stretch or methylenic CH stretch. Experimentally, it has been demonstrated that direct overtone excitation of these modes, e.g., to the fourth excited C-H stretch mode, provides enough energy for the ring-opening to 1,3-butadiene. 

Similarly, in the case of bimolecular reactions, Zare s group [466] confirmed theoretical predictions and demonstrated experimentally [467-469] that by exciting either the OH or the OD bond in HOD one can selectively enhance product forma- tion in a subsequent H + HOD reaction. Specifically, when the OH bond is excited , the reaction yields H2 + OD, whereas when the OD bond is excited, H reacts with HOD to form the HD + OH product. In these experiments, the OH was prepared either by overtone excitation [57, 58] to the fourth vibrational level v = 4 or by excitation to the u = 1 state by Raman pumping [102]. As yet to be verified experimentally is the computational prediction of Manz et al. [124, 125] that strong optimized pulses can also achieve selective excitation of higher lying vibrational) ... 

A number of studies of unimolecular reactions induced by single-photon vibrational overtone excitation have appeared this year. The basic motivation behind such experiments has been the possibility of observing mode-selective photochemistry resulting from the apparently highly localized nature of overtones of X-H stretching vibrations (X = C, O, etc.) in polyatomic molecules. A series of studies by Jasinski, Frisoli, and Moore examined the isomeriza-... 

Figure 51. Schematic depicting the pump-probe-fluorescence detection scheme used to monitor the photodissociation of H202 subsequent to overtone excitation on the ground-state surface.

Ab initio anharmonic IR intensities are calculated using dipole moments estimated on grids by an ab initio method (at the same time when potential energies are computed). For fundamental and overtone excitations ... 

First-principles calculations of anharmonic vibrational spectroscopy of large molecules 187 Table 9.4 OH-stretching overtone excitation frequencies for HNO3... 

Experimental studies have had an enormous impact on the development of unimolecular rate theory. A set of classical thermal unimolecular dissociation reactions by Rabinovitch and co-workers [6-10], and chemical activation experiments by Rabinovitch and others [11-14], illustrated that the separability and symmetry of normal modes assumed by Slater theory is inconsistent with experiments. Eor many molecules and experimental conditions, RRKM theory is a substantially more accurate model for the unimolecular rate constant. Chemical activation experiments at high pressures [15,16] also provided information regarding the rate of vibrational energy flow within molecules. Experiments [17,18] for which molecules are vibrationally excited by overtone excitation of a local mode (e.g. C-H or O-H bond) gave results consistent with the chemical activation experiments and in overall good agreement with RRKM theory [19]. 

The Wittig group has used this method to study the H-atom channels in HNCO and H2CO dissociation [171,172], They have also studied the predissociation of HF(r = 3) prepared by overtone excitation and photodis-... 

Because of the relatively high energy of the initial CH overtone excited state, the stretch-wag potential for the overtone excited CH bond requires special consideration. The stretch-wag potential, through cubic terms, in the input force field in curvilinear coordinates is... 

The Hamiltonian will now be summarized for one overtone excited CH oscillator interacting with the N — 1 ring modes. Normal coordinates for the ring Q2,. . . , Q.v and for the overtone excited oscillator Q, are defined by uncoupling the overtone excited oscillator from the ring. The final form for the Hamiltonian then contains terms (both potential and kinetic) which couple the CH stretch mode to the ring modes. The derivation of the vibrational Hamiltonian was presented in Section II.C of Benzene I (103), and we will only summarize the final result here. The vibrational Hamiltonian may be partitioned into the terms... 

The primitive (or uncontracted) basis set for the vibrational dynamics consists of the direct product of harmonic oscillator functions for all ring normal modes times Morse oscillator functions for the overtone excited CH stretch. Thus for 21-mode planar ben-... 

The initial condition for the dynamics at / = 0 was again chosen to be a v = 3 overtone excited CH oscillator. The subsequent dynamics was then generated by propagating the time-dependent wave function in the active space. If we let C(/) denote the column vector of the expansion coefficients of the various basis functions at time t, then this state vector evolves according to the equation C(t) = U(r)C(r), where U(r) = exp[—iH/] is the time propagator associated with the vibrational Hamiltonian matrix H. In order to evaluate the propagator, we used the Chebyshev expansion (Sec. II.C.3). 

The dissociation has been studied photochemically by exciting the dipole active HN stretch with infrared multiphoton pumping and with one-photon local mode overtone excitation. From the discussions above, it is clear that we are primarily concerned in this chapter with the vibrational events which produce the activation of the N-N reaction coordinate and not with the electronic events which determine whether the curve crossing occurs. Hence, we have focused on the events on the ground electronic surface of the parent molecule which lead to an extension of the N-N bond to 3.5a . 

Figure 46. TOF spectrum transformed to energy transfer distribution for CO/Rh(lll). For greater coverage of CO than in Fig. 45, the (Vs x V3)R30° structure is formed, which allows strong overtone excitations to occur. (Reproduced from Fig. 5 of Ref. 130, with...

In recent experiments lasers have been used to excite unimolecular reactants. In overtone excitation, states in which an MH or MD bond contains n quanta of vibrational energy are prepared by direct single-photon absorption. Here, M is a massive atom in contrast to H or D. For states with large n, the energy in the bond may exceed the molecule s unimolecular threshold. Overtone excitation of CH and OH bonds has been used to decompose molecules (Crim, 1984, 1990). One limitation of this technique is the very weak oscillator strengths of overtone absorptions. 

The spectra in figure 4.16 show that a single atom substitution may have an enormous effect on the intramolecular dynamics, since there is a remarkable difference in the rotationally resolved spectra of the fundamental and first overtone of these two molecules. The line width of the silicon-substituted compound is significantly narrower than that of tert-butylacetylene in both the fundamental and first overtone. There is also a striking different behavior of the two molecules in going from fundamental to overtone excitation. Tert-butylacetylene shows a decreased IVR lifetime in the overtone, dropping by almost a factor of 2. In contrast, the silicon substituted compound shows exactly the opposite behavior since the lifetime in the fundamental is decreased by almost a factor of 2 compared to the overtone. 

A circumvention of this problem is through the use of two-color excitation, or double resonance. An infrared laser can excite a selected rotational state (with a greatly reduced Doppler width because v is much less). As discussed in a following section on overtone excitation, the absorption of the second (visible) photon leads to an excited state in a much more precisely defined energy. 

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