Rotational excitation sources

Three major sources contribute to the rotational excitation of the fragment molecule ... 

Using a seeded nozzle beam source of N02 rather than a thermal beam, it was concluded [260] that the energy disposal is insensitive to reagent rotational excitation, but that there might be a slight increase in rotational excitation of the OH product as the reagent translational energy is increased. 

The dipolar or induced dipolar natime of molecules means that the impacting electron can cause rotational excitation but, because of conservation of momentum, very little of the kinetic energy of the electron can be imparted and little direct vibrational excitation can occur (Cottrell, 1965). Further, although ion-sources frequently operate at fairly high temperatures, the population of vibrationally excited states of molecules even at 500°K is very low and the source of the large vibrational excitation of ions must be sought elsewhere. For illustrative... 

Fluorescence Decay. The fluorescence is mainly from the v = 32 vibrational level of state 5 to various v" levels of the ground state, each consisting of a rotational doublet as discussed in Exp. 39. This emission will be to the red (long wavelength or Stokes) side of the 532-nm excitation source thus an orange or red filter is used to block green and pass red light. It is not necessary to resolve the emission into individual transitions since the decay rate of each is assumed to have the same dependence on the upper B state concentration, I2. 

The photolysis of HBr at 193 nm has been used by Kleinermanns and Wolfrum as a source of translationally hot H atoms in a series of investigations of the elementary reactions of H with O2, H2O, and C02. They have provided a summary of these studies which also includes other laser-induced processes. Nascent vibrational, rotational, and fine-structure state distributions of OH(X n) resulting from the H( S) + 02( Sg ) reaction were presented simultaneously with the results of classical trajectory calculations of the total and state-to-state cross-sections for this reaction performed on an ab initio potential surface. OH vibrational and rotational excitation was found to be high and a preference for the tt component of the A-doublets was noted. The latter was taken to indicate that the reaction takes place in a planar configuration at high collision energies. For H+H2O and H+CO2, similar... 

The plane-polarized light pulses characteristic of mode-locked lasers also provide an ideal excitation source for time-dependent fluorescence depolarization studies although conventional excitation sources can be used. If the rotational relaxation time of the excited molecule is comparable to its fluorescence decay time, then the vertical (I ) and horizontal (Ix) components of the fluorescence decay observed through suitable polarizers following excitation by polarized li t pulses, may be analysed to provide information concerning the size and motion of die molecule and Sect. 5. However, if only the true fluorescence decay characteristics are of interest it is necessary to compensate for these emission anisotropy effects Perhaps the simplest technique is to analyse only that component of fluorescence emitted at 54.7° to the direction of pdarization of the excitation source, the so-called magic-angle ... 

Laser lUman spectroscopy (LRS). The spectra were recorded on a Nicolet 950 FT-Raman spectrometer instrument, equipped with a nitrogen cooled Ge detector. A Nd YAG laser (1064 nm) was used as excitation source. The measurements were performed with a power at the sample of 100-200 mW in order to avoid decomposition and thermal effects. The samples were rotated to provide a noncontinuous irradiation of any given spot on the samples. The spectral slit width was typically 4 cm". ... 

As we outlined in the theoretical section, the time dependence of the fluorescence is critically dependent on the type of exciting source used. We therefore list the experiments as a function of increasing laser width. We will first limit ourselves to the photodynamics of the J = 0, K = 0 rotational state of the lB3u of pyrazine, since only of that state we know the ME spectrum. 

Very little is known about the nature of rotational energy transfer in a collision between an electronically excited molecule and a ground-state atom or molecule. In the few reported studies the experimental method is fundamentally the same as that described at the beginning of Section III.A. An initial rotational distribution is established by narrow-band excitation. The fluorescence emission contour is recorded twice, under collision-free and thermal equilibrium conditions, and then again under conditions such that there is one collision during the lifetime of the excited state. The differences in the rotational contours of the three emission spectra are then used to infer the pathway of rotational energy transfer, and the rate of that transfer. Some examples of the emission spectra recorded under these conditions are shown in Fig. 22. Because of the small spacings between the rotational levels of polyatomic molecules most excitation sources prepare nonthermal superpositions of rotational states rather than pure rotational states, and this complicates interpretation of the observations. 

---