Dip Spectroscopy

Let us consider a laser oscillating at a single frequency (single-mode operation) and gas molecules inside the laser resonator which have absorption transitions at this frequency. Some of the molecules will be pumped by the laser-light into an excited state. If the relaxation processes (spontaneous emission and collisional relaxation) are slower than the excitation rate, the ground state will be partly depleted and the absorption therefore decreases with increasing laser intensity. 

The electromagnetic field in the cavity is a standing wave which can be described as a superposition of two travelling waves propaga- 

In this case only one group with velocity components = 0 Avz is available for absorption of the laser line. The absorption coefficient therefore has a minimum at the center of the inhomogeneous molecular absorption profile (see Fig. 14 b), and the laser intensity will 

If the laser is operated close above threshold this maximum is much more enhanced 

This situation corresponds to the well-known saturation effect in the emission of most gas laser transitions, where, for the same reason, fewer upper-state molecules can contribute to the gain of the laser transition at the center of the doppler-broadened fluorescence line than nearby. When tuning the laser frequency across the doppler-line profile, the laser intensity therefore shows a dip at the centerfrequen-cy, called the Bennet hole or Lamb dip after W.R. Bennet who discovered and explained this phenomen, and W.E. Lamb 2) who predicted it in his general theory of a laser. 

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Natural line broadening is usually very small compared with other causes of broadening. However, not only is it of considerable theoretical importance but also, in the ingenious technique of Lamb dip spectroscopy (see Section 2.3.5.2), observations may be made of spectra in which all other sources of broadening are removed. 

Vj = 1 <— v" = 1 transition will be at a different energy than the Vj = 0 <— v" = 0. We use this fact to measure the vibrational spectrum of V (OCO) in a depletion experiment (Fig. 12a). A visible laser is set to the Vj = 0 Vj = 0 transition at 15,801 cm producing fragment ions. A tunable IR laser fires before the visible laser. Absorption of IR photons removes population from the ground state, which is observed as a decrease in the fragment ion signal. This technique is a variation of ion-dip spectroscopy, in which ions produced by 1 + 1 REMPI are monitored as an IR laser is tuned. Ion-dip spectroscopy has been used by several groups to study vibrations of neutral clusters and biomolecules [157-162]. 

M. Saeki, S. i. Ishiuchi, M. Sakai, and M. Fujii, Structure of 1 naphthol/alcohol clusters studied by IR dip spectroscopy and ab initio molecular orbital calculations. J. Phys. Chem. A 105, 10045 10053 (2001). 

This section will focus on the stmcture and energetics of chiral molecular complexes studied with Fourier-transform IR (FT-IR), microwave, LIF, hole burning (HB), IR fluorescence dip spectroscopy, resonance-enhanced multiphoton ionization (REMPl Fig. 5), and RET spectroscopy. 

Fig. 15. Lamb dip spectroscopy of Left side Stark spectrum (with...

The hyperfine structure of the R(127) rotational line in the 11-5 band of molecular iodine at X = 6328 A could be resolved by Lamb dip spectroscopy with a halfwidth of 4.5 MHz, and the influ-enie of isotopic substition and has been studied 38, 338a)k)... 

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)... 

Lamb dip spectroscopy provides a very sensitive tool for studying small frequency shifts and broadening of spectral lines which normally would be undetectable because they may be small compared to the doppler width. These investigations yield information about collisions at low pressures, where the effect of far distant collisions is not suppressed by the more effective close collisions. This allows the potential between the collision partners at large intermolecular distances to be examined. 

The last two chapters discussed spectroscopic studies which used coincidences between laser lines and transitions in other atoms or molecules. These investigations have been performed either with lasers as external light sources, or inside the laser cavity. In the latter case coupling phenomena occur between the absorbing species and the laser emission, one example of which is the saturation effect employed in Lamb dip spectroscopy and laser frequency stabilization. This chapter will deal with spectroscopic investigations of the laser medium itself and some perceptions one may obtain from it. 

III. Laser-Driven Population Dynamics and Coherent Ion Dip Spectroscopy... 

III. LASER-DRIVEN POPULATION DYNAMICS AND COHERENT ION DIP SPECTROSCOPY... 

Coherent ion dip spectroscopy has been shown to be a versatile tool for the investigation of high-lying intramolecular vibrations in the ground state of molecules and of intermolecular vibrations of van der Waals complexes. 

The IR spectra of carbazole and carbazole-(H20) ( =l-3) clusters in a supersonic jet, measured by IR dip spectroscopy, show vibrational structures of both the monomer and the clusters in the 2900-3800 cm frequency region, assigned to the NH stretch of carbazole and the OH stretches of H2O molecules in the clusters <2001PCA8651>. In the first excited singlet and triplet states A -(4-cyanophenyl)carbazole gives rise to transient bands at 2090 cm and 2060 cm detected by time-resolved infrared absorption spectroscopy and attributed to the CN stretch modes of the molecule <2002CL340>. 

Figure 29.1 (a) Scheme of two-color fluorescence dip spectroscopy. The pump beam excites a molecule from the ground state (S0) to the S i state. Then, the erase beam further excites the St molecule to a higher excited state, S . Due to various relaxation processes from S states, such as internal conversion to the ground state,... 

Watanabe, T., Iketaki, Y., Omatsu, T., Yamamoto, K. and Fujii, M. (2005) Two-point separation in far-field superresolution fluorescence microscopy based on two-color fluorescence dip spectroscopy, Part I Experimental evaluation. Appl. Spectrosc., 59, 868-872. 

Fig. 4. Ion dip spectroscopy schematic. R2PI is used to ionize and mass-select the cluster, Mx of interest. A second laser pulse, prior or coincident to ionization, excites an intermediate level (Sm), which depletes the ground state and reduces the ion signal of...

The use of laser ionization methods offered a more subtle and gentle way to ionize neutral clusters, which could then be mass-analyzed using time-of-flight (TOP) methods. Castleman first introduced these methods to clusters (named ion-dip spectroscopy) and applied to it electronic transitions. The method is depicted in Fig. 4. 

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