Direct 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... 

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... 

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. 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.

For some purposes the V-V processes can be extremely useful, since they provide a means of achieving significant excitation to levels hi above the ground vibrational state without recourse to direct promotion from v 0 in extremely weak overtone bands, or without relying on sequential (i. . v 0 v = 1 v = 2, etc.) or multiphoton pumping. A simple example can be provided as an illustration of this. Consider a harmonic oscillator that is pumped sufficiently strongly on lines in its (1,0) band for 50% of the molecules to be raised instantaneously to the... 

In the preceding sections we discussed cases in which the molecule is first electronically excited to an upper-state PES before it dissociates, in the second step, on this PES. Dissociation can, of course, also take place directly on the ground-state PES, without detour via an excited state. This process is known as unimolecular dissociation and plays an important role in combustion processes or atmospherical chemistry (see Rates of Chemical Reactions and Unimolecular Reaction Dynamics). Experimentally, unimolecular dissociations can be directly measured either by overtone excitation (OH stretching vibration, for example) or by. stimulated emission pumping from an excited electronic state. On the theoretical side, all... 

Overtone and combination bands of gaseous HN3 were investigated in the near-IR and visible ranges between 1070 and 500 nm. 3vi, 4vi, and 5vi were excited conventionally [33], whereas 5vi, 6vi, 7vi, and associated combination bands were excited by direct pumping or by IR-visible double-resonance pumping through the Vi band [34 to 36]. The band origins in cm" are ... 

---