Sub-Doppler resolution

In a skimmed supersonic jet, the parallel nature of the resulting beam opens up the possibility of observing spectra with sub-Doppler resolution in which the line width due to Doppler broadening (see Section 2.3.4) is reduced. This is achieved by observing the specttum in a direction perpendicular to that of the beam. The molecules in the beam have zero velocity in the direction of observation and the Doppler broadening is reduced substantially. Fluorescence excitation spectra can be obtained with sub-Doppler rotational line widths by directing the laser perpendicular to the beam. The Doppler broadening is not removed completely because both the laser beam and the supersonic beam are not quite parallel. 

Figure 3-9. Sub-Doppler resolution OH LIF spectra indicate a modest amount of c.m. translational energy for all of the OH levels monitored in the reaction H + N20 -> OH + N2 see Bohmer et al. (1992) for details. A Franck-Condon projection of the 1,3-hydrogen shift transition state N-N separation of 1.23 A (solid line, lower part) predicts a high degree of N2 vibrational excitation (upper part) and yields good agreement with the data (shaded circles, lower part).

This Doppler width can be avoided by typical sub-Doppler laser spectroscopy techniques. Laser saturation spectroscopy with a resolution close to the natural line width was used for a test of Special Relativity at the ESR. For such sub-Doppler resolution one must also take into account the small additional broadening and shift arising from the angle 0 between laser beam and ion beam in the Doppler formula. At an interaction length of 10 meters and more, angles are easily controlled to be better than 1 mrad. This limits a possible shift, which enters by... 

Sub-Doppler resolution measurements of average OH kinetic energies indicate that this degree of freedom is diminished slightly from the case of bulk conditions, as shown in Figure 15 and Table 2. 

Figure 47. Sub-Doppler resolution OH LIF spectra from the H + N2O -> OH + N2 reaction. Entries (a) and (b) are for N20/HBr under bulk and complexed conditions, respectively (193 nm photolysis). Note the large difference in linewidths, compared to the case shown in Figure 15 for the H + CO2 - OH + CO reaction. Experimental resolution is 0.05cm .

With the sub-Doppler resolution of the Lamb-dip technique, about 500 Stark resonances have been identified for 44 lines of the V2 band and 31 lines of the V4 band [13]. Two Doppler-free lines with signs opposite to those of Lamb dips were observed in a study of the V2 °P(5,3) transition by laser Stark spectroscopy using the 10P(18) CO2 laser line. They were identified as infrared-infrared double resonance transitions, caused by accidental overlapping of the °P(5,3) line in the V2 band with the °P(4,3) line in the hot band 2V2-V2 [14 to 16]. 

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]. 

T.E. Gough, R.E. Miller, G. Scoles, Sub-Doppler resolution infrared molecular beam spectroscopy. Faraday Disc. 71,6 (1981)... 

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 rotational energy levels for both ground and r s = 1 vibrationally excited states of CH3 are shown schematically in Fig. 5.12. At sht expansion temperature of 25K, essentially all of the population collapses into the two lowest spin allowed rotational states. Therefore, a total of 5 transitions are allowed from these two states, which are also shown in Fig. 5.12, i.e., the middle panel shows the one / = 3/2 transition and right most panel shows all four possible 7=1/2 transitions. At sub Doppler resolution, each rovi-brational transition displays additional structure, both varying the number of peaks (up to four distinct features for the (1,0) <— (1,1) transition) as... 

Two new bands — in-phase (ui) and out-of-phase (I ls) antisymmetric CH2 stretching vibrations of allyl radical have been obtained in the slit jet discharge spectrometer, as the sample spectra shown in the top panel of Fig. 5.18. The data have been successfully analyzed with a Watson asymmetric rotor Hamiltonian, yielding precise band origins and rotational constants for both bands. The high quality of least squares fits to ground state combination differences indicates that the rotational level structure in the lower state is well behaved, while the reduced quality of fits to the vibrational transitions, on the other hand, suggest the presence of Coriolis mediated rotational perturbations in the upper state. Due to sub-Doppler resolution (Ai/ 70 MHz) in the slit jet expansion. 

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]. 

No monochromator is needed since the absorption coefficient a(ca) and its frequency dependence can be directly measured from the difference Al(ca) = alp(o)) - Ij(w) between the intensities of reference beam and transmitted beam (Fig.8.lb). The spectral resolution is higher than in conventional spectroscopy. With tunable single-mode lasers it is only limited by the linewidths of the absorbing molecular transitions. Using Doppler-free techniques (see Chap.10) even sub-Doppler resolution can be achieved. 

The Fourier transform of the time-resolved fluorescence intensity I(t) yields its spectral distribution I(o)) with sub-Doppler resolution. Figure 11.24 illustrates as an example quantum beats measured by ANDRA et al. [11.19] in the fluorescence following the excitation of three hfs levels in the 6p P3/2 state of the ion. Either a tunable dye laser crossed perpen-... 

Figure 2 illustrates for the complex NO2 spectrum how much more information can be obtained with sub-Doppler resolution. The upper, Doppler-limited excitation spectrum was taken with a single mode dye laser traversing a cell which contained NO2 gas at a pressure of 0.2 torr. The lower spectrum represents a small section of the upper one, indicated by the bar, recorded with sub-Doppler resolution of about 12 MHz in a collimated supersonic NO2 beam. The triplet structure of the lines reflects the hyperfine structure due to the nuclear... 

Fig. 2. Doppler-limited excitation spectrum of NOg (left spectrum) and a section of this spectrum, marked by the bar, recorded with sub-Doppler resolution in a collimated NO2 beam (from ref. 11).

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