Laser Wavelength Setting

In spectroscopy using continuous laser tuning it is very important to be able to monitor the change in wavelength. For this purpose, a stable multipass interferometer (Sect.6.2.3) can be used from which the fringes are recorded together with the spectrum [9.43]. 

We will now consider different laser spectroscopy methods. We will first discuss techniques in which a reasonable resolution is sufficient and where relatively broad-band lasers (0.1-0.01 A) and Doppler-broadened samples are used. We will then describe how resonance techniques can be combined with laser excitation. In the following sections, time-resolved spectroscopy will be considered. Lifetime measurements and structure determinations using such techniques will be described. In this context a comprehensive survey of methods for determining the radiative properties of atoms and molecules is made. 

While the laser is used essentially as a bright lamp in the above-mentioned contexts, a survey of the most important methods of Doppler-free spectroscopy, employing the extremely small linewidth of a single-mode laser, is made in Sect.9.5. 

We will now consider different laser spectroscopy methods. We will first discuss techniques in which a reasonable resolution is suflScient and where relatively broadband lasers (0.01—0.001 nm) and Doppler-broadened samples 

Extremely high laser intensities can be achieved with the pulsed systems discussed in Sect. 8.7.2. Experiments using such systems are surveyed in Sect. 9.6. A further aspect of the extreme performance achievable with laser techniques is the ultra-narrow bandwidth achievable in CW single-mode laser systems as discussed in Sect. 8.5.1. In order to benefit from this, the Doppler broadening must be eliminated. The different techniques of Doppler-free laser spectroscopy arc discussed in Sect. 9.7. Finally, the ultimate resolution achievable by laser cooling and trapping techniques is discussed in Sect. 9.8. 

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This chapter deals mainly with (multi)hyphenated techniques comprising wet sample preparation steps (e.g. SFE, SPE) and/or separation techniques (GC, SFC, HPLC, SEC, TLC, CE). Other hyphenated techniques involve thermal-spectroscopic and gas or heat extraction methods (TG, TD, HS, Py, LD, etc.). Also, spectroscopic couplings (e.g. LIBS-LIF) are of interest. Hyphenation of UV spectroscopy and mass spectrometry forms the family of laser mass-spectrometric (LAMS) methods, such as REMPI-ToFMS and MALDI-ToFMS. In REMPI-ToFMS the connecting element between UV spectroscopy and mass spectrometry is laser-induced REMPI ionisation. An intermediate state of the molecule of interest is selectively excited by absorption of a laser photon (the wavelength of a tuneable laser is set in resonance with the transition). The excited molecules are subsequently ionised by absorption of an additional laser photon. Therefore the ionisation selectivity is introduced by the resonance absorption of the first photon, i.e. by UV spectroscopy. However, conventional UV spectra of polyatomic molecules exhibit relatively broad and continuous spectral features, allowing only a medium selectivity. Supersonic jet cooling of the sample molecules (to 5-50 K) reduces the line width of their... 

The experiment also yields deactivation cross-sections in addition to results on the photochemical reaction, and it sets an upper limit for continuous absorption in Br2 at the ruby laser wavelength. 

A further improvement and more freedom in the choice of laser wavelengths can be expected with the use of dye vapors. In liquids, the phase-matching concentration is set by the requirement that the anomalous dispersion of the dye compensates for the normal dispersion of the solvent. The latter is a new parameter that can be varied at will in the gas phase by changing the nature and partial pressure of the buffer gas. The broader resonances of dyes as opposed to metal vapors, which are sometimes used for this purpose, is an advantage for tunable frequency tripling of dye lasers. Another advantage results from the possibility of working at much lower temperatures than with metal vapors. 

Figure VIII-2 shows the principle of isotope enrichment by two-photon ioni/.ation of 233U atoms. The excitation wavelength is 4266.275 0.02 A. A band width of 0.1 cm 1 is much narrower than an isotope shift of 0.32cm 1. Since the preferentially excited 235U atoms decay in 10-7sec the second laser source to ionize the excited atoms must be pulsed within 10 - 7 sec. The wavelength of the second laser must be shorter than 3777 A, as the combined photon energy must exceed the ionization potential, 6.187 eV, of U atoms. If the first laser is set at 4266.325 A in coincidence with an absorption line of 238U atoms, an isotopic yield ratio of 3000 1 for 238U/23,U is obtained in comparison with 140 1 for the same ratio in the starting material.

Fig. 8.4 Relative ground state photoionization cross section as a function of laser wavelength in Rb in the presence of a 4335 V/cm field. Note the relative gain and offset settings. For the light polarization parallel to the electric field (lower trace), field dependent resonance structure extends beyond the zero field limit. No structure is observed for the case of light polarized perpendicular to the field (upper trace) (from...

There is no inherent sample size restriction, large or small, but is fixed by the optical components used in the instrument. The diffraction limit of light, roughly a few cubic micrometers depending on the numerical aperture of the optics used and the laser s wavelength, sets the lower bound.7 In a process application, the type of fiber optics used also affects sample volume examined. Macroscopic to microscopic samples can be measured with the appropriate selections of laser wavelength, laser power, and optics. 

Although various interchanges of laser wavelengths, power meters, etc. will be made to control systematics, the basic technique for the measurement is indicated in fig. 5. The lasers are set on laser lines approximately equidistant from, but on either side of the resonance centroid, and balanced in power. The lasers are chopped in anti-phase and the difference signal, S(u> 1) — S(u>2) is recorded. The beam velocity is varied till the zero-crossing (where the signals are equal) is found. The resonance centroid (in the ion s rest frame) is then obtained from the relativistic Doppler formula and the mean of the two laser frequencies. 

Figure 11-2. 1 + 1 REMPI, 1 + 1 REMPI and UV absorption spectra of 1,3-DMU [15]. The 1 + 1 REMPI spectrum was obtained by scanning the pump laser and setting the probe laser at 220 nm with a delay time of 10 ns. Neither REMPI spectrum was normalized by the laser power, and at the short wavelength side of the figure, the low output power of the OPO laser resulted in the missing S3 feature in the 1 + 1 spectrum. The absorption spectrum was taken at 140°C, the same temperature as that of the pulsed valve during the REMPI experiments. (Reproduced with permission from J. Phys. Chem. 2004, 108, 943-949. Copyright 2004 American Chemical Society.)...

In the experiment, the wavelength of the photolysis pulse from the laser was set to successive values which were uniformly spaced across the spectral region studied. Then, for each value of the wavelength of the delayed pulse was tuned... 

Figure la shows the experimental set-up for measurement of outgassed species from fluoropolymers on exposure to 157 nm. The exposure source is a LAMBDA PHYSIK OPTex UV excimer laser (Wavelength 157 nm, Power 0.8 mJ/pulse, Rep. Rate 30 Hz, Pulse duration 5-10 ns FWHM, laser intensity 2 MW/cm2) where a vacuum chamber was connected via CaF2 lens. A PFEIRRER... 

Nevertheless in the case of 248-nm irradiation, the fit with one set of parameters shows agreement with both sets of experimental data. This may be an indication, together with the different threshold behavior as compared to 308-nm irradiation, that there are different mechanisms acting at the different laser wavelengths, and that the concept of designing polymers for special laser wavelengths is working. 

A protein may have more than one metal center, each with its own set of electronic transitions, and in favorable cases the RR spectra can be elicited selectively by judicious choice of laser wavelengths. For example, sulfite reductase has an iron-isobacteriochlorin cofactor, siroheme, which is coupled magnetically to a Fe4S4 cluster, probably through a thiolate bridge. The siroheme modes are strongly enhanced in resonance with the isobacteriochlorin Soret or Q bands, but intermediate excitation (457.9-488.0 nm) enhances the Fe4 4 cluster modes (Fig. 11). The protein rubrerythrin has two rubredoxin-like Fe(Cys)4 centers and a (/u.-oxo)di-iron(III) center (similar to those found in hemerythrins and ribonucleotide reductase), each of which can be enhanced selectively by excitation at... 

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