REMPI spectra

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. 

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...

An extended table of available experimental results for cytosine in given by Roos and coworkers [134], The first experimental peak is at ca. 4.6 and the second at 5.2 eV [132, 135, 136], Gas phase REMPI spectra locate the origin of Si at4eV [97],... 

State-resolved inelastic scattering for a wide range of incident conditions ( ), d,) are measured for this system by combining molecular beam techniques with (2 + 1) ion TOF REMPI detection of the scattered molecules [58]. Energy transfer parallel to the surface is measured from the Doppler broadening of the REMPI spectra. Trapping... 

Figure 2. Rotationally resolved REMPI spectra of the 6 and 6 1 vibronic bands of the benzene-Ar complex. No broadening of the lines in the 6q1 band is observed showing that even for an excess energy of 1444 cm-1, which is more than four times the binding energy, the complex is stable on the nanosecond time scale. (Taken from Ref. 29.)...

A cut along the pump laser frequency axis for a nonresonant dump laser frequency yields REMPI spectra. The same cut for a resonant dump laser frequency show a splitting of the 2 < - 1 transition that can be explained in the intuitive picture of a dynamic Stark effect. (For a closer discussion of this point, see Refs. 6 and 30). A cut along the dump laser frequency... 

In preliminary experiments using the new apparatus we have produced rovibrationally state-selected Hj, CO+, NJ, NH3, and NO+ and have begun to study simple bimolecular reactions, for example, H + H2 - H3 + H at low collision eneigy (<0.5 eV) [29], For studying the H2/H2 reaction, where the molecular ions react with the species that itself is the precursor neutral molecule, the beam consists of neat H2 only, backing pressure of 2 bars. However, for other reactions such as CO+ + H2, the ion precursor and reactant gases are coexpanded from the same nozzle. The translational temperature has been measured (see Section IV.B) to be of order 2 K, and the rotational temperatures, determined from REMPI spectra, are typically in the range 5-20 K (except H2). 

Figure 4 REMPI spectra of desorbed CO for the B1 + (V =0) X1 + (v" = 0) transition (a) (2 + 1)REMPI spectrum from Pt(00 1) [30] and (b) (1 + E)REMPI spectrum from 0 03 (000 1) [29], Solid lines in (a) indicate simulated curves at thermal equilibrium with various temperatures.

Desorption from hep hollow species The desorption from hep hollow species is observed from the surface shown in Fig. 9c. The decay curves of the desorption yield as a function of photon numbers at X = 193 nm observed by Fukutani et al. [8] consists of two components with cross sections of 1 x 10 18 and 1 x 10-20 cm2. The former may be obtained from NO adsorbed on minor defect sites. The REMPI spectra were observed for the slow component and the latter is assigned to be the desorption of the hep hollow species. 

The fluence dependence of the CO and CO+ desorption yield at X = 193 nm reveals that CO and CO+ are desorbed as single-photon and three-photon processes, respectively. From (2 + 1)REMPI spectra of neutral CO, the effective rotational temperature can be estimated to be 130 K for v = 0. The population ratio in the v = 1 state to the v = 0 state is estimated from the relative intensity of the Q band head to... 

The rotational energy distributions by the REMPI spectra of desorbed NO from the hep hollow species on Pt(l 1 1), which is induced by 2.3-6.4 e V laser irradiation, are represented by a Boltzmann distribution such as in Fig. 16a. It is impossible to understand that molecules desorbed by a non-thermal process reach thermal equilibrium in the rotational energy distribution during the very short residence time in the excited state for chemisorbed species on metal and semiconductor surfaces (lifetime = 10 16-10-14 s [62, 72, 73]). Besides, no rotational freedom exists in the chemisorbed state and the desorption is... 

Figure 12-7 shows clusters involving 9-methylguanine, demonstrating how tautomeric blocking can be used to confirm structural assignments [47], Figure 12-8 shows REMPI spectra of cytosine dimers and their methyl derivatives [48], We used... 

Figure 12-8. REMPI spectra of various cytosine dimers. Asterisks indicate origins of different structures as determined by UV-UV double resonance spectroscopy. The corresponding structures, as determined by IR-UV double resonance spectroscopy are shown below...

It is possible that the absence of some of the most stable structures of base pairs in the gas phase is due to short excited state dynamics. Direct experimental proof of this explanation requires fast time resolved measurements. For an indirect measure one can consider the linewidths in UV spectra. Of the 27 base pairs analyzed in the gas phase so far, 24 structures do not form WC type pairs and all of those exhibit sharp structured REMPI spectra, as shown in Figure 12.9(a). Only three... 

HjO t Identification of new HjOCD . Bi) state from (3+1) REMPI spectra 341... 

Figure 5.6 Two REMPI spectra of NO in the A state. The room-temperature spectrum shows a broad rotational distribution while the cold spectrum shows just a few of the rotations. Taken with permission from Miller and Compton (1986).

We also obtained REMPI spectra of doubly functionalized PFPEs of type A2 for a niunber of chain lengths. These spectra exhibit broadening and... 

The combination of resonant laser excitation to an intermediate level and the subsequent mass analysis of the ion also makes the technique species selective. Therefore, one can distinguish REMPI spectra of systems that have the same mass, e.g. phenol-H2 and phenol-CO. On the other hand, care needs to be taken not to approach the REMPI experiment in a brute-force approach, as one may be tempted to increase weak signals by increasing the laser pulse energies. Non-resonant multiphoton ionization (MPI) processes may destroy the carefully adjusted species selectivity, as will be shown in the example of the REMPI investigation of CaH/CaD reaction products see Figure 9.5 below. 

In addition to the high-precision spectroscopic data for a particular complex or product molecule, other valuable information on the chemical reaction process can be extracted from the REMPI spectra, e.g. internal energy-level populations or dynamic features. 

Pereira R, Skowronek S, Gonzalez Urena A, Pardo A, Poyato JML, Pardo AH. 2002. Rotationally resolved REMPI spectra of CaH in a molecular beam . J. Mol. Spectrosc. 212(1) 17-21. 

One of the requirements for recording REMPI spectra with sharp resonances is that the excited electronic state should be sufficiently long-lived to allow for the absorption of the second photon. For the work presented here, pulsed nanosecond... 

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