REMPI

REMPI Resonance-enhanced multi- Laser-induced ionization ... 

Plenary 9. J W Nibler et al, e-mail address niblerj chem.orst.edu (CARS and SRS). High resolution studies of high lymg vibration-rotational transitions in molecules excited in electrical discharges and low density monomers and clusters in free jet expansions. Ionization detected (REMPI) SRS or IDSRS. Detect Raman... 

Plenary 11. W Kiefer et al, e-mail address wolfgang.kiefer mail.imi-wue.de (TR CARS). Ultrafast impulsive preparation of ground state and excited state wavepackets by impulsive CARS with REMPI detection in potassium and iodine duners. 

Figure B2.3.8. Energy-level sehemes deseribing various optieal methods for state-seleetively deteeting ehemieal reaetion produets left-hand side, laser-indueed fluoreseenee (LIF) eentre, resonanee-enlianeed multiphoton ionization (REMPI) and right-hand side, eoherent anti-Stokes Raman speetroseopy (CARS). The ionization oontinuiim is denoted by a shaded area. The dashed lines indieate virtual eleetronie states. Straight arrows indieate eoherent radiation, while a wavy arrow denotes spontaneous emission.

The most widely used of these tecluiiques is resonance-enlianced multiphoton ionization (REMPI) [ ]. A schematic energy-level diagram of the most conunonly employed variant (2 + 1) of this detection scheme is illustrated in the... 

This teclnhque can be used both to pennit the spectroscopic detection of molecules, such as H2 and HCl, whose first electronic transition lies in the vacuum ultraviolet spectral region, for which laser excitation is possible but inconvenient [ ], or molecules such as CH that do not fluoresce. With 2-photon excitation, the required wavelengdis are in the ultraviolet, conveniently generated by frequency-doubled dye lasers, rather than 1-photon excitation in the vacuum ultraviolet. Figure B2.3.17 displays 2 + 1 REMPI spectra of the HCl and DCl products, both in their v = 0 vibrational levels, from the Cl + (CHg) CD reaction [ ]. For some electronic states of HCl/DCl, both parent and fragment ions are produced, and the spectrum in figure B2.3.17 for the DCl product was recorded by monitoring mass 2 (D ions. In this case, both isotopomers (D Cl and D Cl) are detected. 

In the ideal case for REMPI, the efficiency of ion production is proportional to the line strength factors for 2-photon excitation [M], since the ionization step can be taken to have a wavelength- and state-mdependent efficiency. In actual practice, fragment ions can be produced upon absorption of a fouitli photon, or the ionization efficiency can be reduced tinough predissociation of the electronically excited state. It is advisable to employ experimentally measured ionization efficiency line strengdi factors to calibrate the detection sensitivity. With sufficient knowledge of the excited molecular electronic states, it is possible to understand the state dependence of these intensity factors [65]. 

Figure B2.3.17. REMPI spectra of the HCl and DCl products from the reaction of Cl atoms with (CH3)3CD [63], The mass 36 and 2 ion signals are plotted as a fiinction of the 2-photon wavenumber. Assigmnents of the...

In contrast to the ionization of C q after vibrational excitation, typical multiphoton ionization proceeds via the excitation of higher electronic levels. In principle, multiphoton ionization can either be used to generate ions and to study their reactions, or as a sensitive detection technique for atoms, molecules, and radicals in reaction kinetics. The second application is more common. In most cases of excitation with visible or UV laser radiation, a few photons are enough to reach or exceed the ionization limit. A particularly important teclmique is resonantly enlianced multiphoton ionization (REMPI), which exploits the resonance of monocluomatic laser radiation with one or several intennediate levels (in one-photon or in multiphoton processes). The mechanisms are distinguished according to the number of photons leading to the resonant intennediate levels and to tire final level, as illustrated in figure B2.5.16. Several lasers of different frequencies may be combined. 

As an example, we mention the detection of iodine atoms in their P3/2 ground state with a 3 + 2 multiphoton ionization process at a laser wavelength of 474.3 run. Excited iodine atoms ( Pi/2) can also be detected selectively as the resonance condition is reached at a different laser wavelength of 477.7 run. As an example, figure B2.5.17 hows REMPI iodine atom detection after IR laser photolysis of CF I. This pump-probe experiment involves two, delayed, laser pulses, with a 200 ns IR photolysis pulse and a 10 ns probe pulse, which detects iodine atoms at different times during and after the photolysis pulse. This experiment illustrates a frindamental problem of product detection by multiphoton ionization with its high intensity, the short-wavelength probe laser radiation alone can photolyse the... 

Figure B2.5.17. (a) Time-dependent intensity / and redueed fluenee F/Fq for a single-mode CO2 laser pulse used in the IR laser photolysis of CF I. Fq is the total fluenee of the laser pulse, (b) VIS-REMPI iodine atom signals obtained with CO2 laser pulses of different fluenee (after [113]).

For the examples given here, three different instruments have been used. While an EI-MBMS setup and a REMPI-MBMS apparatus are available in our own... 

Kamphus, M. et al., REMPI temperature measurement in molecular beam sampled low-pressure flames, Proc. Combust. Inst., 29,2627, 2002. 

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]. 

REMPI resonance-enhanced multiphoton ionization spectroscopy... 

TD/Py-REMPI-ToFMS an electronic separation between the components of a mixture is achieved. 

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... 

GC-MS has been reviewed [203,204] with particular attention being paid to additive analysis [243] GC-MS interfaces were reported by Oehme [227]. For GC-REMPI-ToFMS, see Section 7.5. Various pertinent monographs are available [227,228,244,245]. 

Increasing reliance on mass spectrometry as the universal detector for GC has not solved all the problems of additive identification. Isomer identification is impossible (except for REMPI technology), but is hardly an issue in additive analysis. 

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]. 

For chemical speciation, X-ray absorption spectroscopy is another supreme tool taking advantage of its electronic and structural information power. Also, REMPI-MS is outstanding in its selectivity for molecular species. Radioanalytical methods have also been used for speciation analysis [548]. Microscopical speciation analysis requires SSIMS or LMMS [549]. 

The use of GC-MS in polymer/additive analysis is now well established. Various GC-based polymer/additive protocols have been developed, embracing HTGC-MS, GC-HRMS and fast GC-MS with a wide variety of front-end devices (SHS, DHS, TD, DSI, LD, Py, SPE, SPME, PTV, etc.). Ionisation modes employed are mainly El, Cl (for gases) and ICPI (for liquid and solid samples). Useful instrumental developments are noticed for TD-GC-MS. GC-SMB-MS is a fast analytical tool as opposed to fast chromatography only [104]. GC-ToFMS is now about to take off. GC-REMPI-MS represents a 3D analytical technique based on compound-selective parameters of retention time, resonance ionisation wavelength and molecular mass [105]. 

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