Overtone pumping

Experimentally one can investigate resonances by various spectroscopic schemes, as indicated in Fig. 1 by direct overtone pumping [11] from the ground vibrational state, by vibrationally mediated photodissociation [12] using an excited vibrational level as an intermediate, or by stimulated emission pumping (SEP) [13-15] from an excited electronic state. In all cases it is possible to scan over a resonance and thereby determine its position j4s aHd its width hkU). A schematic illustration of an absorption or emission spectrum is depicted on the left-hand side of Fig. 1 all of the more or less sharp structures at energies above threshold are resonances. Figure 2 shows an overview SEP spectrum measured for DCO [16]. It consists of... 

Fig. 1.3. Schematic illustration of unimolecular decay induced by electronic excitation. In (a) the photon creates a bound level in the upper electronic state which subsequently decays as a result of a radiationless transition (rt) to the electronic ground state. In (b) overtone pumping directly creates a quantum state above the threshold of the electronic ground state. In both cases the dissociation occurs in the electronic ground state.

Another example of a dramatic difference in the population of the two spin-orbit states is the dissociation of HN3 in the electronic ground state induced by overtone pumping (Foy, Casassa, Stephenson, and King 1988 ... 

A typical overtone pumping experiment, with double-resonance excitation, is illustrated in the left-hand panel of Fig. 10 [126]. It involves three different laser pulses of 5-8 ns duration. The first laser promotes molecules to a specific vibrational-rotational intermediate state with two quanta in the OH stretch vibration. After a delay time the second pulse promotes only the preselected molecules to a higher OH stretch overtone level, that lies in the continuum and dissociates to OH and Cl. Finally, the OH fragments are detected by LIF using a third laser. 

Overtone pumping spectroscopy has the limitation that, because excitation starts in the ground vibrational state, only very specific states can be accessed, e.g., the (ui,0,0) progression in HOCl and only few other states in the vicinity of the (ui,0,0) states. Stimulated emission pumping (SEP) [147], on the other hand, involves a transition to an excited electronic state, whose equilibrium geometry may be quite different from the equilibrium in the ground state. Therefore, Pranck-Condon factors are comparatively large for a wide variety of vibrational states, not just the... 

Experimental investigations of the state-specific dissociation of HOCl are limited to the pure overtone bright states (ui,0,0) which, as discussed in 3.2, can be accessed by direct overtone pumping. Apart from very few... 

Figure 3.9 shows the resonance-mediated reaction mechanism. The HF(v = 3)-H VAP on the new PES is very peculiar with a deeper vibrational adiabatic well close to the reaction barrier and a shallow van der Waals (vdW) well. The ID wave function for the ground resonance state in Fig. 3.9 shows that this state is mainly trapped in the inner deeper well of the HF(v = 3)-H VAP with a considerable vdW character, whereas the excited resonance wave function is mainly a vdW resonance. Because of the vdW characters, these two resonance states could likely be accessed via overtone pumping from the HF(v = 0)-H vdW well. 

Vibrational overtone spectra are obtained using an infrared pump-ultraviolet probe technique. The OH-Ar complexes are prepared with two quanta of OH stretch (vqh=2) by direct overtone pumping using tunable infrared radiation at 1.4 pm ( 7000 cm" ). The infrared is generated by Raman shifting (second Stokes) the output of a NdrYAG pumped dye laser. The OH-Ar (vqh=2) complexes are then probed by ultraviolet laser-induced fluorescence on OH-Ar transitions located near the OH 1-2 transition, as outlined above. 

Fig. 4.6 Schematic diagram showing the resonance-mediated reaction mechanism for the F - - H2 reaction with two resonance states trapped in the peculiar HF(n = 3)-H VAP well. The ID wave functions of the two resonance states are also shown. The (003) state is the ground resonance state the (103) resonance is the first excited resonance state. Calculated van der Waals states for the lower VAPs are also shown. OP, overtone pumping ffb, barrier height Ec. coUision energy...

With broad-band pulses, pumping and probing processes become more complicated. With a broad-bandwidth pulse it is easy to drive fundamental and overtone transitions simultaneously, generating a complicated population distribution which depends on details of pulse stmcture [75], Broad-band probe pulses may be unable to distinguish between fundamental and overtone transitions. For example in IR-Raman experiments with broad-band probe pulses, excitation of the first overtone of a transition appears as a fundamental excitation with twice the intensity, and excitation of a combination band Q -t or appears as excitation of the two fundamentals 1761. 

Post, R. L. (1999). Active transport and pumps. In Membrane Permeability Vol. 48 100 Years Since Ernest Overton, eds. Deamer, D. W., Kleinzeller, A. and Fambrough, D. M. Academic Press, San Diego pp. 397-417. 

The initial step in the double-resonance scheme is the excitation of a local mode hydrogen stretch vibration localized in a hydrogen halide moiety. In principle, this can be done either at the fundamental or one of the overtones. With presently available Ti sapphire lasers and parametric oscillators (OPOs), it is possible to saturate fundamentals and first overtones, thus ensuring maximum population transfer. Second overtones cannot be pumped as efficiently, but offer enormous discrimination against background and can be used to shift frequencies out of the vacuum ultraviolet and into a more user-friendly part of the ultraviolet. Thus, first and second overtones are very attractive. 

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) ... 

We elected to study coherent up-pumping dynamics in solution-phase metal-hexacarbonyl systems because of their strong vibrational infrared absorption cross sections, relatively simple ground-state spectra, and small (ca. 15 cm ) anharmonic overtone shifts. It was felt that these systems are ideal candidates to demonstrate that population control could be achieved for polyatomic species in solution because the excited state population... 

With the Ti sapphire system, it became possible to see all relevant VER processes in NM 48. Some representative data are shown in Fig. 14, where the pumped 3000 cm 1 C-H stretch decays with a 2.6 ps time constant. C-H stretch excitation is also associated with excitation in the first overtone of the antisymmetrical C-H bend and the antisymmetrical N02 stretch. As the C-H stretch decays, energy builds up with a 2.6 ps time constant in the symmetrical C-H bending and N02 stretching vibrations at 1400 cm, as shown in Fig. 14a. These daughter vibrations decay with a 15 ps time constant. Looking to lower energy vibrations such as the C-H... 

It would help if the e-bend overtone could be frequency-shifted away from the fundamentals. That would be possible if the e-bend were inho-mogeneously broadened (see Section IV.G.). Then we could pump the C-H stretch around 3000 cm-1 and excite only bend overtones in the - 4500 cm 1 region, away from the region between the two fundamentals. Figure 20c,d shows the results of pumping ACN at 3000 cm-1. At t = 0, a new peak is observed in the C-H bend spectrum near 1500 cm-1, whose spectral width is about the same as the apparatus-limited bandwidth. This... 

On the political overtones of natural philosophy, see Stephen Shapin and Simon Schaffer, Leviathan and the Air-Pump (Princeton University Press, 1985). 

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