Rotationally excited species, determination

The formation of vibrationally excited products is nearly always energetically possible in an exothermic reaction, and these products can be detected by observing either an electronic banded system in absorption or the vibration-rotation bands in emission. In principle, rotational level distributions may be determined by resolving the fine structure of these spectra, but rotational energy is redistributed at almost every collision, so that any non-Boltzmann distribution is rapidly destroyed and difficult to observe. In contrast, simple, vibrationally excited species are much more stable to gas-phase deactivation and the effects of relaxation are less difficult to eliminate or allow for. 

For characterizing a dipolar molecule in its electronic ground state, few methods are more instructive than pulsed-nozzle Fourier-trans-form microwave spectroscopy (32). As illustrated schematically in Fig. 5, a short pulse of microwave radiation directed at the gas pulse excites a rotational transition in the species of interest subsequently the rotationally excited molecules reemit radiation, which is detected. This technique provides a remarkably sensitive probe for transients, the properties of which can be specified with all the precision and detail peculiar to rotational spectroscopy only microseconds after their production. In relation to a weakly bound adduct A --B formed by two molecular reagents A and B, for example, we may draw on the rotational spectrum to determine such salient molecular properties as symmetry, radial and angular geometry, the intermolecular stretching force constant and internal dynamics, the electric charge distribution, and the electric dipole and quadrupole moments of A -B (see Table I). 

It has to be kept in mind that the translational and internal energy distributions of the desorbing species are of great importance. Therefore, typical detection tools are TOFMS to derive velocity and angular distributions, and either LIF or REMPI are used to determine internal energy distributions. Both LIF and REMPI provide information on the vibration-rotation excitation of photodesorbing species or fragments. 

Therefore, if a number of rotational constants is determined for isotopie species in the ground as well as excited vibrational states, these anharmonic potential constants may be determined simultaneously with the re stmctural parameters [28], The geometric significance of A, and Ce (Eq. (4)) and is unambiguous. However, because of the various experimental difficulties, aeeurate stmetures are currently known for only a small number of simple molecules, as listed in the following tables. 

In contrast with previous studies on He2Cl2 cluster, in the present work localized structures are determined for the lower He2Br2 vdW states. Traditional models based on a He2Cl2 tetrahedron frozen stucture have failed to reproduce the experimental absorption spectrum, suggesting a quite delocalized structure for its vibrationally ground state. Here, based on ab initio calculations we propose different structural models, like linear or police-nightstick , in order to fit the rotationally resolved excitation spectrum of He2Cl2 or similar species. 

For reasons discussed later (Section II, E) the ground state of a stable organic molecule is almost always totally symmetrical and it is then possible, at least in principle, to determine the excited state symmetry species from the rotational structure of the electronic bands. The basis for the decision lies in a symmetry that exists between the conditions for the production and absorption of light waves. According... 

The spectrum of radiation from electronically excited states of atoms appears as lines, when the emission from a hot gas is diffracted and photographed, whereas radiation from these excited states of molecules appears as bands because of emission from different vibrational and rotational energy levels in the electronically excited state. Equation (26) shows that the intensity of radiation from a line or band depends upon the temperature and concentration of the excited state and the transition probability (the rate at which the excited state will go to the lower state). Since the temperature term appears in the exponential, as the temperature rises the exponential term approaches unity, as does the ratio of the concentration of the excited (emitting) state to the ground state (as T approaches oo, Ng = Nj). The concentrations of both the ground and excited states, however, reach a maximum, and then decrease due to the formation of other species. The line or band intensity must also reach a maximum and then decrease as a function of temperature. This relationship can be used to determine the temperature of a system. 

---