Fixed-Frequency Lasers

Meanwhile hfs splittings and quadiupole moments of many atoms and molecules have been measured with lamp-dip spectroscopy, using fixed frequency lasers 338b—c) j. tunable dye lasers 338)... 

One common use of SHG is to convert the output of a fixed-frequency laser into a different spectral region. For example, the Nd-YAG laser operates in the near IR at a wavelength of 1,064 nm, while SHG is routinely used to convert the wavelength of the radiation to 532 nm. For x processes, the conversion efficiency can be up to 30% for phase-matched case with a nanosecond laser pulse. 

G. Gerber The reported experiment was done with fixed-frequency laser pulses for the pump and the probe laser. I do agree that the observation of ZEKE electrons and the dynamics of the signal using tunable femtosecond laser pulses would be of interest. 

Most SHG studies involve incident energies in the visible or near-infrared spectrum. Infrared SHG studies are hindered by the current lack of sufficiently sensitive IR detectors. However, the sum frequency generation (SFG) technique allows one to obtain surface-specific vibrational spectra. In SFG, two lasers are focused on the sample surface, one with a fixed frequency in the visible and one with a tunable range of IR frequencies. The sample surface experiences the sum of these frequencies. When the frequency of the infrared component corresponds to a molecular vibrational mode, there is an increase in the total SHG signal, which is detected at the visible frequency [66]. The application of such... 

In fast beams optical excitation has proven to be most useful. Since the fast beams are low in intensity, but continuous, cw lasers have been used. Usually, fixed frequency lasers have been used since fine tuning can be done using the Stark shift or the Doppler shift of the fast beam. The Doppler shift can be used either by changing the angle at which the laser beam and fast beam cross, or by altering the velocity of the fast beam. An early example was the use of the uv line of an Ar laser to drive transitions from the metastable H 2s state to the 40 < n < 55 np states.27 In this particular case the velocity of the beam was changed to tune different np states into resonance. 

A CARS experiment has recently been done to determine the amount of vibrational and rotational excitation that occurs in the O2 (a- -A) molecule when O3 is photodissociated (81,82). Valentini used two lasers, one at a fixed frequency (266 nm) and the other that is tunable at lower frequencies. The 266 nm laser light is used to dissociate O3, and the CARS spectrum of ( (a A), the photolysis product, is generated using both the fixed frequency and tunable lasers. The spectral resolution (0.8 cm l) is sufficient to resolve the rotational structure. Vibrational levels up to v" = 3 are seen. The even J states are more populated than the odd J states by some as yet unknown symmetry restrictions. Using a fixed frequency laser at 532 nm (83) to photolyze O3 and to obtain the products 0(3p) + 02(x3l g), a non-Boltzmann vibrational population up to v" = k (peaked at v" = 0) is observed from the CARS spectrum. The rotational population is also non-Boltzmann peaked at J=33, 35 33, 31 and 25 for v" = 0,1,2,3, and k, respectively. Most of the available energy, 65-67%, appears in translation 15-18% is in rotation and 17-18% is in vibration. A population inversion between v" = 2 and 3 is also observed. 

He-Ne laser. The frequency of the shifted infrared beam is locked to this reference Fabry-Perot cavity whose length is fixed. By changing the acousto-optic modulation frequency, which is provided by a computer-controlled frequency synthesizer, we can therefore precisely control the dye laser frequency over a range of 250 MHz centered at any desired frequency. 

Electronically excited species are generally produced by direct excitation or as the products of photolysis. In addition to conventional light sources, fixed frequency or tuneable visible and ultraviolet lasers are regularly used for single photon excitation or photolysis, as well as... 

High resolution Raman spectra are obtained by scanning the frequency of one of the Raman excitation sources and monitoring the ionization signal as a function of the frequency difference between this laser and the second, fixed frequency, pump laser. 

To acquire an SFG vibrational spectrum of adsorbate molecules on a metal catalyst, two (picosecond) laser pulses are spatially and temporally overlapped on the sample (Fig. 5). One input beam is in the visible range at fixed frequency (covis), and the second one is tunable in the mid-IR region (giir) to probe the vibrational... 

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