Velocity-modulation spectroscopy

The analysis of absorption spectra in molecular gas discharges is by no means simple, because the gas discharge produces a large variety of neutral and ionized fragments from the neutral parent molecules. The spectra of these different species may overlap, and often an unambiguous identification is not possible if the spectra are not known. An elegant technique, developed by Saykally et al. [6.104,6.105], is very helpful in distinguishing between spectra of ionized and neutral species. 

A typical experimental arrangement [133] is depicted in Fig. 1.46. With specially designed electron-switching circuits, polarity modulation frequencies up to 50 kHz can be realized for gas discharges of 300 V and 3 A [134]. The attainable signal-to-noise ratio is illustrated by Fig. 1.47, which shows the band head of a vibrational band of the A 77. 

This technique was first applied to the infrared region where many vibrational-rotational transitions of ions were measured with color-center lasers or diode 

1 Doppler-Limited Absorption and Fluorescence Spectroscopy with Lasers 

A modification of this velocity-modulation technique in fast ion beams is discussed in Sect. 4.5. 

---

Stephenson and Saykally [05Ste] have studied the influence of pressme and Doppler broadening upon the line shapes of pure rotational transitions in the terahertz region using velocity modulation spectroscopy (J = 5<—4, 3070392.8 MHz 7= 3<-2, 1845789.0 MHz). 

Fan et al. [98Fan] have measured fundamental and hot band absorption lines of Si CE, Si CE, SE CE, and Si CE using infrared velocity modulation spectroscopy (IR veloc. Mod.) between 630 and 700 cm . The following parameters have been determined (in cm if not otherwise indicated) ... 

Focl Focsa, C., Dufour, C., Pinchemel, B., Bachir, I.H., Huet, T.R. Laser velocity modulation spectroscopy of the A(3d4s) — X 0(3cf) visible system ofTiCt and characterization of the spin-orbit structure, J. Chem. Phys. 106 (1997) 9044-9049. 

Hal Halfen, D.T., Ziurys, L.M. Velocity modulation spectroscopy of molecular ions I The pure rotational spectrum ofTiCt (X r), J. Mol. Spectrosc. 234 (2005) 34-40. 

Ste Stephenson, S.K., Saykally, R.J. Velocity Modulation Spectroscopy of Ions, Chem. Rev. (2005) 3220-3234. 

Fig. 1.45 Opposite signal phases of the derivative line profiles of negative and positive ions in velocity-modulation spectroscopy [132]...

For the spectroscopy of atoms or ions in gas discharges, optogalvanic spectroscopy (Sect. 1.5) is a very convenient and experimentally simple alternative to fluorescence detection. In favorable cases it may even reach the sensitivity of excitation spectroscopy. For the distinction between spectra of ions and neutral species velocity-modulation spectroscopy (Sect. 1.6) offers an elegant solution. 

For velocity-modulation spectroscopy an ac voltage of 2 kV peak-to-peak and / = 1 kHz is applied along a discharge tube with 1 m length in the z-direction. 

J.W. Farley, Theory of the resonance hneshape in velocity-modulation spectroscopy. J. Chem. Phys. 95, 5590 (1991)... 

In particular, the various applications of laser spectroscopy in physics, chemistry, biology, and medicine, and its contributions to the solutions of technical and environmental problems are remarkable. Therefore, a new edition of the book seemed necessary to account for at least part of these novel developments. Although it adheres to the concept of the first edition, several new spectroscopic techniques such as optothermal spectroscopy or velocity-modulation spectroscopy are added. 

---