Lasers, wavelength tuneable

Laser based on a titanium-doped sapphire crystal. The typical power is a few watts, the wavelength tuneable from about 720 to 960 nm. The pulse width is in the range of 100 fs to a few ps, the repetition rate between 78 and 92 MHz. 

The grating should be tuned to the zero order and the zero point adjusted to ensure agreement. The grating should be tuned to the laser wavelength and the software calibration adjusted accordingly (in cases where the laser line is fixed rather than tuneable, this step is performed upon installation). 

Figure 1C. Additional excitation of the molecular ions takes place at increased laser intensities, where photon absorption (in the ion and fragment ion manifold) and dissociation processes to fragment ions follow each other. Owing to this ladder switching the degree of fragmentation is tuneable from soft ionization (mainly or solely molecular ions) to hard ionization (mainly small or even atomic fragment ions). At appropriate laser wavelengths and intensities metastable decay processes may dominate the mass spectrum. Multiphoton excitation and dissociation is also possible with delayed laser pulses and at different positions in space, giving rise to new forms of tandem mass spectrometry.

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... 

Detection limits in the lOOfg range can be obtained with a tuneable UV laser working at a wavelength of maximum absorption for the compounds of interest. Continuous supersonic beams require high gas loads and combination with a pulsed ionisation technique (e.g. REMPI) is unfavourable in terms of sensitivity. Pulsed valves are a better approach for a GC-UV-MS interface [1021]. 

Harmonics are tuneable when the primary laser radiation is obtained from a tuneable laser, such as a titanium-doped sapphire (Ti S) unit However, the need for wavelength-dependent adjustments in the CPA chain can make the tuning procedure tedious. The laser system described in Sect. 2.2 is easier to tune. Since CPA techniques are not used the entire system tunes easily together with the DFDL oscillator. The peak power is still enough for this system to generate low... 

Methyl salicylate was placed in a rare gas matrix at 4.2 K. A tuneable dye laser was used to excite the sample, while a monochromator was used to measure the emission spectrum. By tuning the dye laser it was possible to measure the excitation as well as the fluorescence spectrum of methyl salicylate. A partially resolved vibrational progression was observed both in the emission and in the excitation spectra. A fluorescence lifetime of 12 ns was measured at all of the emission wavelengths. As expected deutera-tion changed the vibrational progressions. It also lengthened the fluorescence lifetime. From the vibronic structure it was argued that a double minimum potential does not exist in the excited states. An estimate of... 

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