Laser Frequencies

Some recent advances in stimulated desorption were made with the use of femtosecond lasers. For example, it was shown by using a femtosecond laser to initiate the desorption of CO from Cu while probing the surface with SHG, that the entire process is completed in less than 325 fs [90]. The mechanism for this kind of laser-induced desorption has been temied desorption induced by multiple electronic transitions (DIMET) [91]. Note that the mechanism must involve a multiphoton process, as a single photon at the laser frequency has insufScient energy to directly induce desorption. DIMET is a modification of the MGR mechanism in which each photon excites the adsorbate to a higher vibrational level, until a suflBcient amount of vibrational energy has been amassed so that the particle can escape the surface. 

While a laser beam can be used for traditional absorption spectroscopy by measuring / and 7q, the strength of laser spectroscopy lies in more specialized experiments which often do not lend themselves to such measurements. Other techniques are connnonly used to detect the absorption of light from the laser beam. A coimnon one is to observe fluorescence excited by the laser. The total fluorescence produced is nonnally proportional to the amount of light absorbed. It can be used as a measurement of concentration to detect species present in extremely small amounts. Or a measurement of the fluorescence intensity as the laser frequency is scaimed can give an absorption spectrum. This may allow much higher resolution than is easily obtained with a traditional absorption spectrometer. In other experiments the fluorescence may be dispersed and its spectrum detennined with a traditional spectrometer. In suitable cases this could be the emission from a single electronic-vibrational-rotational level of a molecule and the experimenter can study how the spectrum varies with level. 

The second issue concerns molecular specificity. For a simple measurement of SHG at an arbitrary laser frequency, one caimot expect to extract infomiation of the behaviour of a system with several possible adsorbed species. To make the technique appropriate for such cases, one needs to rely on spectroscopic infomiation. In the simplest iiiiplementation, one chooses a frequency for which the nonlinear response of tlie species of interest is large or dominant. As will... 

In the discussion in Section 9.1.6 of harmonic generation of laser radiation we have seen how the high photon density produced by focusing a laser beam into certain crystalline materials may result in doubling, tripling, etc., of the laser frequency. Similarly, if a laser beam of wavenumber Vl is focused into a cell containing a material which is known to absorb at a wavenumber 2vl in an ordinary one-photon process the laser radiation may be absorbed in a two-photon process provided it is allowed by the relevant selection rules. 

Dye lasers, frequency doubled if necessary, provide ideal sources for such experiments. The radiation is very intense, the line width is small ( 1 cm ) and the wavenumber may be tuned to match any absorption band in the visible or near-ultraviolet region. 

Fig. 1. Pumping methods for lasers where is the pump light frequency and is the laser frequency, wavy lines represent radiationless transitions, and the dashed line collisions (a) optical pumping in three-level systems (b) optical pumping in four-level systems (c) pumping by electron impact and...

Fig. 40. Flole-burning spectra of thioindigo in benzoic acid crystal at 1.35 K. The scanning laser frequency cu is measured with respect to the burning laser frequency cUb is detuning of the burning laser frequency relative to the center of absorption line.

Fig. 25. Room temperature Raman spectra for purified single-wall carbon nanotubes excited at five different laser wavelengths, showing evidence for the resonant enhancement effect. As a consequence of the ID density of states, specific nanotubes (n, m) are resonant at each laser frequency [195].

In the following sections, we first show the phonon dispersion relation of CNTs, and then the calculated results for the Raman intensity of a CNT are shown as a function of the polarisation direction. We also show the Raman calculation for a finite length of CNT, which is relevant to the intermediate frequency region. The enhancement of the Raman intensity is observed as a function of laser frequency when the laser excitation frequency is close to a frequency of high optical absorption, and this effect is called the resonant Raman effect. The observed Raman spectra of SWCNTs show resonant-Raman effects [5, 8], which will be given in the last section. 

The third common level is often invoked in simplified interpretations of the quantum mechanical theory. In this simplified interpretation, the Raman spectrum is seen as a photon absorption-photon emission process. A molecule in a lower level k absorbs a photon of incident radiation and undergoes a transition to the third common level r. The molecules in r return instantaneously to a lower level n emitting light of frequency differing from the laser frequency by —>< . This is the frequency for the Stokes process. The frequency for the anti-Stokes process would be + < . As the population of an upper level n is less than level k the intensity of the Stokes lines would be expected to be greater than the intensity of the anti-Stokes lines. This approach is inconsistent with the quantum mechanical treatment in which the third common level is introduced as a mathematical expedient and is not involved directly in the scattering process (9). 

Frequency noise. If the two interferometer arms are not exactly symmetric (if they have different lengths / 0 or finesses()F 0), then the laser frequency noise Ujy can mimic a gravitational wave signal... 

At 10Hz in a typical Nd-YAG laser 1000Hz/- /Hz, and the typical finesse asymmetry is of the order of one percent. In order to detect a gw signal the laser frequency noise has to be lowered by six orders of magnitudes (compared to the noise of a free running laser), and the two arms made as identical as possible. In order to achieve this complex frequency stabilization methods are employed in all interferometric detectors, and in order to insure the perfect symmetry of the interferometer, all pairs of Virgo optical components are coated during the same run (both Fabry-Perot input mirrors then both end mirrors are coated simultaneously). 

The present study demonstrates that the analytic calculation of hyperpolarizability dispersion coefficients provides an efficient alternative to the pointwise calculation of dispersion curves. The dispersion coefficients provide additional insight into non-linear optical properties and are transferable between the various optical processes, also to processes not investigated here as for example the ac-Kerr effect or coherent anti-Stokes Raman scattering (CARS), which depend on two independent laser frequencies and would be expensive to study with calculations ex-plictly frequency-dependent calculations. 

Fig. 8. Examples of some of the donor-acceptor substituted TEEs prepared for the exploration of structure-property relationships in the second- and third-order nonlinear optical effects of fully two-dimensionally-conjugated chromophores. For all compounds, the second hyperpolarizability y [10 esu], measured by third harmonic generation experiments in CHCI3 solution at a laser frequency of either A = 1.9 or 2.1 (second value if shown) pm is given in parentheses. n.o. not obtained...

First, let us consider a selective and complete excitation in a three-level problem by quadratically chirping the laser frequency as shown in Fig. 28 [42] (the field parameter F is the laser frequency oa). The energy separation CO23... 

Figure 29. Complete excitation from 11 > to 2 > by one period of frequency chirping in the case of the three-level model. Upper part-time variation of the population. Middle part-time variation of laser frequency. Bottom part-envelope of the laser pulse. Taken from Ref. [42].

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