Laser photon wavelengths

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... 

Since our main objective was to remove all the chlorine and hydrogen atoms from the polymer chain, C-PVC films were further exposed to the UV radiation of the medium pressure mercury-lamp. This led to a dark brown material w.hich was found to be unable to carry an electrical current, even after extended irradiation time. Therefore we turned to a powerful laser source, a 15 W argon ion laser tuned to its continuous emission at 488.1 nm. At that wavelength, the degraded polymer film absorbs about 30 % of the incident laser photons. The sample was placed on a X-Y stage and exposed to the laser beam at scanning rates in the range of 1 to 50 cm s, in the presence of air. 

Fig. 6.12. Data obtained by the single-photon wavelength 340 nm observation wavelength timing technique using a mode-locked ion- 390 nm. Reference scattering solution argon laser that synchronously pumps a cavity- (Ludox). Number of channels 900 channel dumped dye laser. Sample solution of POPOP width 4.68 ps. Result t = 1.05 + 0.01 ns in cyclohexane (undegassed). Excitation x = 1.055.

Many molecules which have absorption bands in the wavelength region of existing laser lines can be excited by absorption of laser photons into single isolated rotational-vibrational levels of the electronic ground state 1W>-103) (jn the case of infrared laser lines) or of an excited electronic state (with visible or ultraviolet lines)... 

Figure 11.18 illustrates the principles of application of REMPI to NO (discussed in more detail later). The electronically excited states of NO are shown in Fig. 11.18a and some potential ionization schemes in Fig. 11.18b (Pfab, 1995). Pulsed tunable lasers with wavelengths from 190 to 1000 nm and spectral resolutions of 0.1 cm 1 are readily available. To ionize NO, the absorption of two, three, or four photons is needed. The first photon excites the NO into an intermediate state from which it is ionized using a second or, in some cases, two more photons. The transitions are described as an (n + m) transition, where n is the...

Figure 1. Versions of photoionization spectroscopy wherein not only the dependence of the multiphoton ionization efficiency on the laser wavelength is subject to measurement, but also the mass spectrum of photons and energy spectrum of photoelectrons (a) energy-level diagram (b) collision of a neutral particle with laser photons.

ZEKE-PES was pioneered by Miiller-Dethlefs, Sander and Schlag18,25. They and others26 recorded the zero kinetic energy photoelectrons produced by absorption of one or more photons to a resonant ion state as a function of pulsed laser excitation wavelength. ZEKE electrons were extracted by an electric field pulse which permitted a delay between the creation and collection of electrons. During this delay, non-ZEKE electrons departed the focal volume, leaving only ZEKE electrons for collection by the extraction pulse. 

A photon wavelength of 266.2 nm is used in the experiments reported here, the fourth harmonic of a special neodymium laser system which can generate powerful ultra-violet pulses of a joule or more. " Experimental conditions are (see also table 1) molecular beam density at the interaction volume, 6x 10 molecules cm laser energy, 0.3 J per pulse (4x 10 photons per 10 ns pulse, i.e., 30 MW) laser repetition rate, one pulse per minute and flight path length, 5.63 cm. 

Table 1 Laser emission wavelengths obtained by two-photon pinnping at 790 nm for different molecules...

The signal intensity was stronger on Si than on quartz. Since CI2 gas has only a weak absorption (e 5 Zcm mol ) at this wavelength, some enhancement by the Si wafer must be present. This enhancement may be caused by energy transfer from the Si substrate excited electronically by the laser photons. The underlying Si wafer can absorb UV photons quite well. 

The CH emission accounts for 1% of the total observed emission and requires 12.76 eV of energy which compares to the 12.84 eV provided by two ArF laser photons. It is interesting to note that Metzger and Cook observed visible emission in the 300-600 nm region as a function of acetylene photolysis wavelength between 103 and 58 nm. Between 103 and 92 nm there is a sudden onset of visible fluorescence. This region corresponds very closely to the thermodynamic threshold for... 

A notable exception regarding cost and complexity is the method of resonant laser ablation (RLA). In RLA, a low-fluence pulsed laser beam is focused onto the solid sample (Eiden et al., 1994). The laser pulse first ablates or desorbs a small amount of sample. On the timescale of the laser pulse (typically a few nanoseconds), the atoms liberated from the surface absorb laser photons and are ionized. By using a laser wavelength resonant with an atomic transition in the atoms of interest, ionization is highly selective. This method has been extensively demonstrated with relatively low-cost YAG pumped dye lasers. 

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