Excitation Doppler-limited

Doppler-free and Doppler-limited fluorescence spectroscopic studies of the He Is2p-ls3d transitions Calculation of electron excitation rate constants for formation of He(ls2s, - S) and He(ls2p, - p ) using evaluated experimental and theoretical data... 

Tunable diode laser spectroscopy has been employed in order to observe the Zeeman effect in the i.r. absorption of molecules with no electromagnetic moment, due to differences between the excited- and ground-state g-factors. Doppler-limited resolution was obtained for and CHjDI in the region 820—... 

The demands of experimental equipment are much less stringent than those necessary for complete resolution and analysis of absorption spectra of the same molecule. This advantage still remains if a few upper levels are simultaneously populated under Doppler-limited excitation [160],... 

The different sensitive techniques of Doppler-limited laser spectroscopy discussed in the previous sections supplement each other in an ideal way. In the visible and ultraviolet range, where electronic states of atoms or molecules are excited by absorption of laser photons, excitation spectroscopy is generally the most suitable technique, particularly at low molecular densities. Because of the short spontaneous lifetimes of most excited electronic states E, the quantum efficiency tjk reaches... 

The three-photon absorption can be used for the excitation of high-lying molecular levels with the same parity as accessible to one-photon transitions. However, for a one-photon absorption, lasers with a wavelength A/3 have to be available in order to reach the same excitation energy. An example of Doppler-limited collinear three-photon spectroscopy is the excitation of high-lying levels of xenon and CO with a narrow-band pulsed dye laser at X = 440 nm (Fig. 2.40). For one-photon transitions light sources at A. = 146.7 nm in the VUV would have been necessary. 

Particularly for polyatomic molecules with their complex visible absorption spectra, the reduction of the Doppler width is essential for the resolution of single lines [392]. This is illustrated by a section from the excitation spectrum of the SO2 molecule, excited with a single-mode frequency-doubled dye laser tunable around X = 304 nm (Fig. 4.4b). For comparison the same section of the spectrum as obtained with Doppler-limited laser spectroscopy in an SO2 cell is shown in Fig. 4.4a [391]. 

Fig. 4.4 Section of the excitation spectrum of SO2 (a) taken in a SO2 cell at 0.1 mbar with Doppler-limited resolution (b) taken in a collimated SO2 beam [391]...

The main part of the book presents various applications of lasers in spectroscopy and discusses the different methods that have been developed recently. Chapter 6 starts with Doppler-limited laser absorption spectroscopy with its various high-sensitivity detection techniques such as frequency modulation and intracavity spectroscopy, cavity ring-down techniques, excitation-fluorescence detection, ionization and optogalvanic spectroscopy, optoacoustic and optothermal spectroscopy, or laser-induced fluorescence. A comparison between the different techniques helps to critically judge their merits and limitations. 

Fig. 5.56a,b. Discontinuous tuning of lasers (a) part of the neon spectrum excited by a single-mode dye laser in a gas discharge with Doppler-limited resolution, which conceals the cavity mode hops of the laser (b) excitation of Na2 lines in a weakly collimated beam by a single-mode argon laser. In both cases the intracavity etalon was continuously tilted but the cavity length was kept constant... 

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).

---