Stark-shifted laser lines

Instead of tuning the laser line, one can also shift the absorption lines across the laser line by Zeeman or Stark effects. This is especially advantageous in the far-infrared region where the tuning range of laser lines is restricted. 

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. 

The dominant uncertainties in the measurement were the dc Stark shift of the deuterium line in the discharge tube, the ac Stark shift of the Ps line, the second-order Doppler shift of the thermal Ps and the amplified laser frequency shift relative to the cw laser. Counting statistics show up in the measurement of the Ps - tellurium frequency shifts as one extrapolates to zero laser power and zero atomic velocity. 

Analogously to the LMR technique, Stark spectroscopy utilizes the Stark shift of molecular levels in electric fields to tune molecular absorption lines into resonance with lines of fixed-frequency lasers. A number of small molecules with permanent electric dipole moments and sufficiently large Stark shifts have been investigated, in particular, those molecules that have rotational spectra outside spectral regions accessible to conventional microwave spectroscopy [146]. 

Figure 7.23 illustrates the Stark switching technique, which can be applied to all those molecules that show a sufficiently large Stark shift [908]. In the case of Doppler-broadened absorption lines the laser of fixed frequency initially excited molecules of velocity v. A Stark pulse, which abruptly shifts the molecular ab-... 

I have chosen to include only those lines whose production requires what may be termed standard techniques . Electrically excited and optically pumped lasers are included, except those involving multi-photon systems, tunable pump lasers, or stark-shifted [1.16] far-infrared lasing. These are all useful techniques but beyond the purposes of this book. Gas-dynamic lasers [1.17], chemical lasers and molecular beam masers [1.18] are also excluded. 

What has not been included As mentioned, the lines of CO2 and N2O lasers themselves are not included. They are very numerous, and are already catalogued in a number of references (see Chapter 5). TEA-laser pulsed lines, or any others with pulse lengths less than 1 /iS, are not included. Stark-shifted and multiple photon lines are excluded, as are molecular beam masers and lasers relying on chemical reactions for their excitation. 

Stark pulse on, but radiates when it is off, the emission is Stark shifted from the laser and four heterodyne beat frequencies are produced at the detector, each beat being due to two transitions M M. The four lines are 170 KHz wide and are spaced at 0.83 MHz [11.50]. The exponential decay of the echo signal with increasing delay time t, which is plotted in Fig.11.31, samples the homogeneous dipole dephasing rate due to elastic and inelastic collisions (see Sect.11.4.1). This technique therefore allows measurement of collision rates separately for different M levels. Since different velocity groups of molecules can be sampled, depending on the Stark shift, information on the velocity dependence of the collision rates can also be obtained (see Chap.12). 

B. Friedrich and D. R. Herschbach recognized that molecular axis orientation would exhibit characteristic spectra. They used laser-induced fluorescence spectroscopy to measure Stark shifts (splitting of lines by an electric field) in ICl, showing that this technique can be used to orient the ICl molecule, in spite of the fact that it is a nonsymmetric top molecule. 

The main applications covered in this study are the accurate determination of rotation and rotation-vibration molecular energies the determination of the molecular geometry of simple molecules the evaluation of force field and of the vibration- and rotation-vibration interactions the measurement of pressure broadening and pressure shift of the spectral lines the determination of electric dipole moments via laser-Stark spectroscopy the studies of intramolecular dynamics the calculation of rate constants, equilibrium constants and other thermodynamic data the evaluation of relaxation times. 

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