Sub-Doppler excitation spectroscopy

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. 4.2 Laser spectrometer for sub-Doppler excitation spectroscopy in a collimated molecular...

PURELY ROTATIONAL COHERENCE AND SUB-DOPPLER SPECTROSCOPY. Guided by the theoretical decay simulations of Fig. 46, the first unambiguous observation of thermally averaged rotational coherence effects was made for excitation and detection of the S, - S00° band of jet-cooled t-stilbene.47 Observed fluorescence decays are shown in Fig. 47 theory and experiment match very well. The recurrences associated with rotational coherence effects in fluorescence have been observed for a number of other species as well. Among these species are t-stilbene-, 2, t-stilbene-argon complexes,48 and t-stilbene-he-lium complexes.71 The recurrences allow the determination of the excited-state rotational constants to a high degree of accuracy. [For example, for t-stilbene we find j(B + C) to be 0.00854 + 0.00004 cm-1.] The indications are that with currently available temporal resolution, rotational coherence effects should be observable in a multitude of species and should allow the accurate determination of such species excited-state rotational constants. 

When the beam of a single-mode laser passes in the z-direction through a molecular absorption cell, only molecules with velocity components Vz = 0 y are excited if the laser is tuned to the center frequency of an absorption line with the homogeneous width y. The fluorescence collected within a narrow cone around the z-axis then shows sub-Doppler linewidths, which may be resolved with Fourier transform spectroscopy (Fig. 1.54) [163]. 

Because of sub-Doppler resolution, quantum-beat spectroscopy has been used to measure fine or hyperflne structure and Lamb shifts of excited states of neutral atoms and ions [870]. 

H.S. Schweda, G.K. Chawla, R.W. Field, Highly excited, normally inaccessible vibrational levels by sub-Doppler modulated gain spectroscopy. Opt. Commun. 42, 165 (1982)... 

A ZEKE spectrum is thus acquired by recording the yield of electrons, produced by PFI, when the photo-excitation laser is scanned across successive ionization thresholds. The resolution obtainable by ZEKE spectroscopy is of the order 10 —10 cm , which is governed by the line width of pulsed, tuneable lasers with CW lasers, even sub-Doppler resolution is now achievable. This is sufficient to resolve rotational structure, even with many polyatomic species, and thus enable the determination of molecular ion structures. 

The first experiments on Doppler-free two-photon spectroscopy were performed on the alkali atoms [7.43-7.47] because their two-photon transitions can be induced by cw dye lasers or diode lasers in convenient spectral ranges. Furthermore, the first excited P state is not too far away from the virtual level in Fig. 7.26b. This enlarges the two-photon transition probabilities for such near-resonant transitions. Meanwhile, there are numerous further applications of this sub-Doppler technique in atomic and molecular physics. We shall illustrate them by a few examples only. 

Stark spectroscopy with constant electric fields and tunable lasers has been performed in molecular beams at sub-Doppler resolution to measure the electric dipole moments of polar molecules in excited vibrational states [6.102]. 

Figure 10.2 shows a typical experimental arrangement for sub-Doppler spectroscopy in molecular beams. The photomultiplier PM 1 monitors the total fluorescence Ip- (XL) as a function of the laser wavelength (excitation spectrum, see Sect.8.2), whereas PM 2 records the dispersed fluorescence spectrum excited at a fixed laser wavelength, where the laser is stabilized... 

Our studies of the effect of velocity-changing collisions in an rf-laser double resonance experiment contribute to a new vista into the role of collisictis in laser spectroscopy of sub-level structures the limitation of the observation time of the active atoms due to narrow-bandwidth optical excitation and simultaneous velocity diffusion can be of importance for a variety of spectroscopic techniques that use a velocity-selective excitation and detection of either sublevel populations or sublevel coherence. On the other hand, the collisional velocity diffusion of sublevel coherence within an optical Doppler distribution can also give rise to new and surprising phenomena as will discussed in the next section. 

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