REMPI-PES

Resonance-enhanced MPI, when combined with PES to yield REMPI-PES, permits determination of the electronic and vibrational states in which the ions are created. Since the final ionic state distribution is largely determined by the resonant state, this distribution provides information about the electronic and vibrational character of the resonant state. The dynamic processes which occur in the excited states can also be elucidated. In... 

Rydberg States. Vibrationally and rotationally resolved, (2 +1)-resonance-enhanced multiphoton ionization (REMPI) spectra and REMPI-PES of NH, ND(a A) and NH, ND(X S ) radicals in the UV (cf. pp. 83/7) revealed the existence of some singlet and triplet Rydberg states. Using literature data for the spectroscopic constants of the a and states,... 

Values in parentheses represent the probable uncertainty in interpolating between the Ne calibration lines. - One standard deviation in parentheses. - Hot bands from a A, v=1 (and v = 2 for j A of ND) were also observed. - From a wavelength scan of the total electron current in REMPI-PES study [9]. - From analysis of hot bands. 

NH" <-NH Transitions. The REMPI-PES technique allows selection of a single rovibra-tional level (v, N, J ) in the resonant Rydberg state so that only a few levels of the ion (v, are accessed this procedure substantially simplifies the PES (compared with... 

In the case of the intermediate j state, transitions into X 11, v =0 to 2 levels showing a clear Av = 0 propensity were also observed. For the j A, v = 0, 1, and 2 intermediates however, the spectra demonstrate the excitation of NH (X rir, v =0 to 2, 0 to 3, and 0 to 5), respectively, in non-Franck-Condon vibrational branching ratios and also show some weak probability to form NH (A Z ) ions and a high probability of forming NH (B A) ions, the latter with a clear Av = 0 preference (unpublished results of rotationally resolved REMPI-PES following excitation via higher J levels of the j state [18] are included in the analysis) [10]. 

Resonance-enhanced multiphoton ionization (REMPI) of NH and ND and detection of the NH or ND ions with a time-of-flight (TOF) mass spectrometer [3 to 8] or TOF detection of the photoelectrons (REMPI-PES) [5, 9, 10] enabled highly excited states of the NH (ND) radical (cf. pp. 83/7) and vibrational and rotational populations of the resulting NH or ND ions to be characterized. 

Beside ions, the detection of electrons is possible. If electrons are analysed according to their kinetic energy the result is a new type of photoelectron spectroscopy, called REMPI-PES. This technique usually uses TOF analysers. Prolongation of the flight time of... 

Using (1 + 1) REMPI, the quadrupole alignment Aq2) (E, v, J, Ts = 925 ) has also been measured for D2 associative desorption from Cu(lll) [229] and some of the results are given in Figure 3.24. Aq2) favors helicoptering alignment, in agreement with a PES that favors dissociation of molecules preferentially parallel to the surface (and detailed balance). It increases substantially with J but decreases with Ef. The... 

ADE = adiabatic detachment energies ESC A = electron spectroscopy for chemical analysis HOMO = highest occupied molecular orbitals MAES = metastable atom electron spectroscopy MIES = metastable ionization electron spectroscopy OAT = oxygen atom transfer PES = photoelectron spectra PEI = pulsed field ionization PIES = Penning ionization electron spectroscopy QM = quantum-mechanical REMPI = resonantly enhanced multiphoton ionization SC = semiclassical VDE = vertical detachment energies XPS = x-ray photoelectron spectroscopy ZEKE = zero electron kinetic energy Cp = cyclopentadienyl, Ph = phenyl, CeHs Tp =... 

Conventional photoelectron spectroscopy uses a rare-gas discharge lamp to produce radiation at the wavelength of the He 2p <— Is atomic transition (hu = 21.218 eV). Synchrotron radiation is now widely used for PES because its photon energy is widely tunable yet monochromatic. The initial state, in the first PES experiments, has been the molecular ground state but now, by exploiting Resonance Enhanced Multi-Photon Ionization (REMPI) excitar tion/detection schemes (see Section 1.2.2.3), any excited state of the molecule can be used as the initial state for PES (for a review, see Pratt, 1995). 

Removal of the Iti electron from NH(X S ) leads to the ionic ground state X removal of the 3a electron to the excited ionic states a A B A, and C Only a few experimental data for the first, third, and fourth ionization potentials Ej of gaseous NH are available. Resonance-enhanced multiphoton ionization (REMPI) of NH coupled with photoelectron spectroscopy (PES) yielded the most accurate results so far [1] and confirmed the values for the first E, obtained by electron-impact mass spectrometry (EIMS) [2] and by He I PES of NH [3]. Values for the second and third Ej to be observed in the He I PES of NH were predicted [3] from the optical emission spectra of NH [4]. Adiabatic and vertical Ej s (in eV) are compared in the following table ... 

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