Beam, molecular, splitting

If the laser beam is split into two partial beams which pass into opposite directions through the LMR cell, Doppler-free saturation spectra can be realized (see Sect. 2.2). This allows one to resolve even complex spectra of radicals or neutral molecules. The narrow spectral width of the Lamb-dips facilitates the determination of collisional broadening and the measurement of molecular transition moments. 

The relative populations n(v., J. in the supersonic beam can be measured in the following way. Part of the laser beam is split into a glass cell containing Na-Na2 vapor at thermal equilibrium. When the laser frequency is tuned through the Na2 absorption lines, the laser-induced fluorescence both from the cell and from the crossing point of laser beam 1 with the molecular beam is monitored simultaneously. Since the fluorescence intensity at nonsaturating laser intensities is proportional to the population densities in the absorbing levels, the relative populations... 

The monochromatic laser beam is split into two beams 1 and 2, both of which cross the molecular beam perpendicularly but at different locations = A and 2 = 8 (see Fig.10.10). When the laser frequency is tuned to a molecular transition (vIj, JIj - V, J ), the molecules passing through the pump beam 1 are optically pumped and there are nearly no molecules left in the depleted level (vIjjJ . ). This means that the fluorescence excited by the probe beam 2 is very low. If the pump beam 1 is interrupted for a short time interval At (e.g., by a fast mechanical chopper) the molecules can pass during this interval At without being pumped. Because of their different velocities they reach the probe beam 2 at different times t = L/v, where L = 22 - The time-resolved fluorescence intensity induced by the cw probe beam therefore reflects the velocity distribution of molecules in the level (vV,JV). 

In dimers composed of equal molecules the dimer components can replace each other through tunneling. This effect has been discovered by Dyke et al. [1972] as interconversion splitting of rotational levels of (HF)2 in molecular beam electric resonance spectra. This dimer has been studied in many papers by microwave and far infrared tunable difference-frequency laser spectroscopy (see review papers by Truhlar [1990] and by Quack and Suhm [1991]). The dimer consists of two inequivalent HE molecules, the H atom of one of them participating in the hydrogen bond between the fluorine atoms (fig. 60). PES is a function of six variables indicated in this figure. 

The systems so far described have all been single-beam spectrometers. As in molecular spectrometry, a double-beam spectrometer can be designed. This is shown diagrammatically in Fig. 2.13. The light from the source is split into two beams, usually by means of a rotating half-silvered mirror or by a beam splitter (a 50%-transmitting mirror). The second reference beam passes behind the flame and, at a point after the flame, the two beams are recombined. Their ratio is then electronically compared. 

The preceding estimate is based on one-fermion theory, so the observed resonance frequency in a fermion beam may be different as a result of fermion-fermion interaction. Therefore, it is strongly advisable that I be tunable over a wide range to search for the actual resonance pattern. The same experiment can then be repeated in a proton, atomic or molecular beam and the RFR effect should be // 2-dependent with a pattern of resonance determined by the novel chemical shift factor Spin-spin interaction between fermions would split the spectrum as in ordinary NMR, but the RFR fingerprint would be unique. 

Inversion splitting of the vibrational spectrum of ammonia has been used to create the first molecular microwave amplifier (maser) [86, 87]. The inversion population in the ammonia maser is achieved by transmission of the molecular beam through a non-homogeneous electric field. Ammonia molecules in symmetric and antisymmetric states interact with the electric field in different ways and they are therefore separated in this field. They are then directed to the resonator. 

The resolution of the molecular beam experiments is high enough to observe even rather small nuclear hyperfine interactions such as the spin-spin and spin-rotation interactions as well as the larger quadrupole coupling interactions. The largest terms in the Hamiltonian for the hyperfine splittings are given below 66) ... 

Nuclear hyperfine splittings in the rotational spectra of dimers have been observed in the molecular beam electric resonance experiments and the Fourier transform microwave experiments. In most cases, the coupling constants are interpreted with the simplified expression given in Eqn. (6) for axially symmetric molecules in the K=0 rotational manifold. Thus both the nuclear quadrupole coupling term and the... 

There are several other important aspects of the experiment which should be mentioned. The waveguide cell is surrounded by a solenoid coil which can produce a magnetic field parallel to the ion beam direction the magnitude of this field (up to 50 G) is often sufficient to produce observable Zeeman splittings which greatly assist spectroscopic assignment, as we will see. ft is also possible to expose the molecular ion beam to two different microwave frequencies this so-called double resonance technique enables two different microwave transitions to be connected, if they share a... 

Isolated tellurium clusters 48-50 (Tex n = 7-9) were produced in a supersonic molecular beam and their vacuum-UV-photoelectron spectra were recorded at a photon energy of hv = 8.3 eV by a photoionization-photoelectron-photoion triple coincidence method. The trimer and tetramer were obtained as stable species in the tellurium cluster beam, unlike sulfur and selenium. The spectra of the odd-membered tellurium clusters have a tendency to be split and broadened, in contrast to those of the seven-membered clusters. For the clusters with n > 5, the spectra of tellurium clusters are similar to those of selenium counterparts, which may suggest a resemblance to the geometric structures between small tellurium and selenium clusters (see Chapters 14.09 and 14.10) <2002MI337>. A similiar study has been conducted by Curtiss and co-workers on selenium clusters <1998CPL(287)282>. 

Ammonia was the first molecule for which the effect of the molecular inversion was studied experimentally and theoretically. Inversion in ammonia was subsequently found to be so important that this molecule played an important role in the history of molecular spectroscopy. Let us recall, for example that microwave spectroscopy started its era with the measurements " of the frequencies of transitions between the energy levels in the ground vibronic state of NH3 split by the inversion effect. Furthermore, the first proposal and realization of a molecular beam maser in 1955 was based on the inversion splittings of the energy levels in NH3. The Nobel Prize which Townes, Basov and Prochorov were awarded in 1964 clearly shows how important this discovery was. Another example of the role which the inversion of ammonia played in the extension of human knowledge is the discovery of NH3 in the interstellar space by Cheung and his co-workers in 1968, by measuring the... 

Sutherland predicted an observable (140 kHz) tunneling inversion in the ground vibrational state of PH3, on the basis of their calculated inversion barrier of 6000 cm . However, subsequent quantum chemical calculations have predicted [see a much higher barrier (between 10 000 and 14 000 cm ). A molecular-beam electric resonance spectrometer has been used to measure the ground state inversion splitting in PH3. It was found that the inversion splitting must be lower than the resolution of the spectrometer (1 kHz). Similarly, in a hi -resolution infrared study of the 41 2 band of PH3, Maki et aL found that the splitting of this level must be less than 0.02 cm". ... 

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