Spectroscopy in Collimated Molecular Beams

If the source diameter is small compared with the slit width b and if b d (which means e l), the flux density behind the slit B is approximately constant across the beam diameter, since cos 1 for d l. For this case the density profile of the molecular beam is illustrated in Fig.10.lb. 

The density n(v)dv of molecules with velocities v = v inside the interval V to V + dv in a molecular beam at thermal equilibrium which effuses 

---

Along with the other sub-Doppler techniques, it has the advantage of high spectral resolution, which is mainly limited by the residual Doppler width due to the finite angle between the pump beam and the probe beam. This limitation corresponds to that imposed to linear spectroscopy in collimated molecular beams by... 

Various techniques of sensitive absorption spectroscopy, including nonlinear techniques, which allow a spectral resolution below the Doppler width are described first. These techniques are termed sub-Doppler-spectroscopy and include linear spectroscopy in collimated molecular beams, nonlinear saturation and polarization spectroscopy, and Doppler-free two-photon spectroscopy. Emission spectroscopy, which covers laser-induced fluorescence as well as stimulated emission methods, is described next. The assignment of complex molecular... 

A possible arrangement for saturation spectroscopy in a molecular beam is depicted in Fig. 9.18. The laser beam crosses the molecular beam perpendicularly and is reflected by the mirror Ml. The incident and the reflected beam can only be absorbed by the same molecules within the transverse velocity group = 0ibyA if the laser frequency (o = o)o y matches the molecular absorption frequency coo within the homogeneous linewidth y. When tuning the laser frequency col one observes narrow Lamb dips (Fig. 9.19) with a saturation-broadened width y at the center of broader profiles with a reduced Doppler-width cAcod, from the collimation ratio c 1 of the molecular beam (Sect. 9.1). 

Optothemial spectroscopy is a bolonietric method that monitors the energy in a stream of molecules rather than in the light beam. A well collimated molecular beam is directed toward a liquid helium cooled bolometer. There will be energy... 

There are several aspects of laser spectroscopy performed with molecular beams that have contributed to the success of these combined techniques. First, the spectral resolution of absorption and fluorescence spectra can be increased by using collimated molecular beams with reduced transverse velocity components (Sect. 4.1). Second, the internal cooling of molecules during the adiabatic expansion of supersonic beams compresses their population distribution into the lowest vibrational-rotational levels. This greatly reduces the number of absorbing levels and results in a drastic simplification of the absorption spectrum (Sect. 4.2). 

Fig. 4.1 Laser excitation spectroscopy with reduced Doppler width in a collimated molecular beam (a) schematic experimental arrangement (b) collimation ratio (c) density profile n x) in a collimated beam effusing from a point source A...

A typical laser spectrometer for sub-Doppler excitation spectroscopy in a collimated molecular beam is shown in Fig. 4.2. The laser wavelength Xl is controlled by a computer, which also records the laser-induced fluorescence /fK l). Spectral regions in the UV can be covered by frequency-doubling the visible laser frequency... 

Fig. 4.5 Experimental setup for sub-Doppler spectroscopy in a collimated molecular beam. Photomultiplier PMl monitors the total undispersed fluorescence, while PM2 behind a monochromator measures the dispersed fluorescence spectrum. The mass-specific absorption can be monitored by resonant two-color two-photon ionization in the ion source of a mass spectrometer...

Fig. 4.24 Experimental setup for saturation spectroscopy in a collimated molecular beam...

Level-crossing spectroscopy with lasers has some definite experimental advantages. Compared with other Doppler-free techniques it demands a relatively simple experimental arrangement. Neither single-mode lasers and frequency-stabilization techniques nor collimated molecular beams are required. The experiments can be performed in simple vapor cells, and the experimental expenditure is modest. In many cases no monochromator is needed since sufficient selectivity in the excitation process can be achieved to avoid simultaneous excitation of different molecular levels with a resulting overlap of several level-crossing signals. 

Ezekiel and coworkers [923] used the reduction of the Doppler width in a collimated molecular beam (Sect. 4.1) for accurate heterodyne spectroscopy. The beams of two argon lasers intersect the collimated beam of I2 molecules perpendicularly. The laser-induced fluorescence is utilized to stabilize the laser onto the centers of two hfs components of a visible rotational transition. The difference frequency of the two lasers then yields the hfs splittings. 

F. ByUcki, G. Persch, E. Mehdizadeh, W. Demtroder, Saturation spectroscopy and OODR of NO2 in a collimated molecular beam. Chem. Phys. 135, 255 (1989)... 

Fig. 10.19. OODR spectroscopy in a collimated molecular beam with overlapping pump and probe beam and detection at the sum frequency (intermodulated fluorescence) [10.39]...

---