Design and calibration of an electrostatic energy analyzer-time-of-flight mass spectrometer for measurement of laser-desorbed ion kinetic energies

The design of a hybrid electrostatic energy analyzer-time-of-flight mass spectrometer for measurement of ion kinetic energies produced by laser desorption ionization is presented. The need for experimental evaluation of the calibration and performance of the instrument is discussed and a novel laser multiphoton ionization technique, which allows experimental calibration of the energy bandpass of the electrostatic energy analyzer, is described. Laser multiphoton ionization at varying electric field strengths also allows the effects of electric field distortions on energy resolution of the instrument to be probed. Measurement of the translational energies of ions produced by 266-nm laser desorption ionization at 48 mJ/cm² of material adsorbed to a stainless steel probe by using this instrument also is presented. Ion translational energies of +19±5, +10±5, and +10±5 eV are found for adsorbed Na⁺, K⁺, and m-xylene M⁺, respectively.     

Modeling of thermal and chemical non-equilibrium in a laser-induced aluminum plasma by means of a Collisional-Radiative model

A 0D numerical approach including a Collisional-Radiative model is elaborated in the purpose of describing the behavior of the nascent plasma resulting from the interaction between a 4 ns/65 mJ/532 nm Q-switched Nd:YAG laser pulse and an aluminum sample in vacuum. The heavy species considered are Al, Al⁺, Al²⁺ and Al³⁺ on their different excited states and free electrons. The translation temperatures of free electrons and heavy species are assumed different (T_e and T_A respectively). Numerous elementary processes are accounted for as electron impact induced excitation and ionization, elastic collisions, multiphoton ionization and inverse Bremsstrahlung. Atoms passing from the sample to gas phase are described by using classical vaporization theory so that the surface temperature is arbitrarily limited to values less than the critical point one at 6700 K. The laser flux density considered in the study is therefore moderate with a fluence lower than 7 J cm^? 2.This model puts forward the major influence of multiphoton ionization in the plasma formation, whereas inverse Bremsstrahlung turns out to be quasi negligible. The increase of electron temperature is mainly due to multiphoton ionization and T_e does not exceed 10,000 K. The electron induced collisions play an important role during the subsequent phase which corresponds to the relaxation of the excited states toward Boltzmann equilibrium. The electron density reaches its maximum during the laser pulse with a value ≈ 10²², 10²³ m^? 3 depending highly on the sample temperature. The ionization degree is of some percents in our conditions.     

Electric quadrupole transition rates for even parity levels of Si I

Non-relativistic Hartree-Fock radial wavefunctions have been used to provide ab initio estimates of E2 transition rates from levels of the 3p ² configuration to those of the 3p4f configuration in Si I. Where it is possible to make comparison with experimental data the results are in reasonable qualitative accord with recent multiphoton ionization observations.     

Two-color excitation of NO in a supersonic free jet. Autoionization of high rydberg states

High Rydberg states of NO above the ionization limit have been measured for the isolated molecule in a supersonic free jet by two-color multiphoton ionization. Three Rydberg series (ns, np and nf) were identified, which appeared by rotational and the vibrational autoionization. The rotational structures of the 13s(υ = 1), 13p(υ = 1) and 12f(υ = 1) states have been analyzed in detail. The fluorescence dip spectra for the intermediate A²Σ⁺(3sσ) state have been measured simultaneously and the cross sections of the one-photon absorption to the high Rydberg states from the A²Σ⁺(υ = 1) state have been determined.     

The line profiles of single rotational lines of the H 2 B - X(3, 0) band in an intense infrared light field

The first investigation of the lineshape of single rotational lines of the H ₂ B - X(3, 0) band in an intense infrared light field (λ = 1064 nm) at intensities between 10¹⁰ W/cm² and 10¹² W/cm² is reported. The B - X(3, 0) band is excited with low intensity vacuum ultraviolet radiation (λ ≈ 106 nm). B-state excitation is observed by multiphoton ionization and dissociation of H ₂ with H ⁺ as final product. The lineshapes of the B-X(3, 0) band behave differently in both decay channels. This indicates that they branch before the molecule is photoionized. The intensity dependence of the lineshapes seems to show that at certain intensities in the focused infrared light beam the AC-Stark effect induces transient resonances into the multiphoton excitation process starting at the B(v = 3, J) rotational levels. A qualitative analysis of the states which may influence the B(v = 3, J) multiphoton excitation rate is given.     

Microwave ionization and excitation of Ba Rydberg atoms

We have investigated ionization and excitation of the Ba 6s n s ¹ S ₀ and 6s n d ^1,3 D ₂ series in strong microwave fields. The observed microwave ionization threshold fields, scaling as 0.28n ^?5, and the state mixing fields cannot be completely explained in terms of a single cycle Landau-Zener model. However, by taking into account multiphoton resonant transitions driven by many cycles of the microwave field we have been able to interpet the data. In particular, multi-photon transitions have been found to be responsible for apparent resonance structures and for the unexpectedly low mixing fields. Not surprisingly, doubly excited valence states introduce irregularities into both the microwave ionization and the state mixing field values.     

Multiphoton ionization mass spectroscopic detection of ammonia clusters

Ammonia cluster ions (NH₃)_n?2NH⁺₄ (n = 2?8), havevbeen detected by multiphoton ionization (MPI) mass spectroscopy, incorporating both a supersonic nozzle source and an ArB excimer laser. The MPI method reveals that the intensity ratio of NH⁺₄/NH⁺₃ is more than an order of magnitude greater than that in the electron impact mass spectrum.     

Electronic shell structure in multiply charged silver clusters

Silver clusters are generated by standard laser vaporization technique and ionized via multiphoton ionization. Time-of-flight mass spectrometry reveals singly, doubly and triply charged clusters, Ag _n ^z+ (z=1,2,3). The spectra show, for all charge states, intensity variations, indicating enhanced stabilities for cluster sizes with closed electronic configurations in accord with the spherical jellium model.     

Multiphoton dissociation and ionization of nickelocene

In this paper we report the results of an experimental study of collision-free molecular multiphoton dissociation (MPD) and molecular multiphoton ionization (MPI) of nickelocene (NiC₁₀H₁₀), induced by the light of a tunable dye laser in the wavelength region 3750–5200 A. The spectral dependence of the ion signal reveals a multitude of narrow (fwhm from <0.5 cm^?1 to 1.5 cm^?1) intense peaks superimposed on a very weak background (relative amplitude ratio for peaks/background ≈ 10³). The sharp resonances in the ion signal are attributed, on the basis of spectroscopic analysis, to two-photon resonant three-photon ionization of Ni(I) and to one-photon resonant three-photon ionization of Ni(I), the Ni(I) being produced by MPD of nickelocene. The ion signal in the spectral range 3750–3950 A reveals enhanced continuous background due to MPI of nickelocene. This ion signal spectra, together with studies of the intensity dependence of the overall (nickelocene MPD) - (Ni(I) MPI) processes, as well as the (nickelocene molecular MPI) reaction, reveal four reactive processes. (a) Two-photon molecular MPI for hω ? 3.10 eV photons. (b) Three-photon molecular MPI for hω = 3.10-2.10 eV. (c) Two-photon MPD at hω ? 2.86 eV. (d) Three-photon MPD for hω = 2.8-1.9 eV. The overall dissociation energy of nickelocene (Nicp₂) to give Ni + 2cp was determined to be 5.76 ± 0.60 eV and the two-photon ionization potential of this molecule is 6.29 ± 0.015 eV. Our results provide dynamic evidence concerning a simultaneous “explosive” photodissociation mechanism of Nicp₂ by process (c) and for the dominating role of the dissociative channel, characterized by a branching ratio of ?50 in favor of predissociation over autoionization, for process (c) at 6.3–6.6 eV. The MPD processes (c) and (d) are expected to exhibit intramolecular erosion of phase coherence effects. Processes (c) and (d) are of high efficiency ≈0.01 ions/molecule at saturation exhibited at laser power of ≈ 10⁸ W cm^?2.     

Laser-induced fluorescence spectroscopy of ionic clusters between organic cations and inert gases

The Laser-induced fluorescence spectra of ionic cluster C₆F₆⁺·X_n, where X = He, Ne, or Ar, are obtained by multiphoton ionization of C₆F₆ seeded in an inert gas expansion. Spectral information concerning the red-shifts and linewidths of the ionic cluster are reported and observations relevant to their formation mechanisms are offered.     

A 1Π ← χ1Σ+ resonance-enhanced multiphoton ionization of jet-cooled CO

We report the observation of two-photon resonance enhancement in the four-photon tunable dye laser-induced multiphoton ionization spectrum of CO. The observed structure is readily assigned to the rotational bands of the A ¹Π ← χ¹Σ⁺ (3,0) transition of CO.     

Production of triplets and cold radicals with a pulsed electric discharge in a pulsed supersonic bea]tl

It is found that a pulsed electric discharge in a pulsed supersonic expansion is an efficient source of rotationally cold triplet metastables and radicals. The spectra of both triplet argon (³P_2,0) and carbon monoxide (a ³ II,) as well as CCl radical (X²II) were recorded with mass selective resonant multiphoton ionization detection.     

Surface analysis by laser desorption of neutral molecules with detection by fourier-transform mass spectrometry

Molecules chemisorbed on a platinum single crystal are desorbed by a pulsed laser, ionized by an electron beam or multiphoton ionization, and detected by Fourier-transform mass spectrometry (F.t.m.s.). Laser desorption of ethylene, methanol, cyanogen, benzene and naphthalene is described. In all cases, molecular ions are the major peaks observed in the mass spectra, and the minor peaks correspond to known electron-impact fragments of the adsorbates. For the systems investigated thus far, the laser-desorption F.t. mass spectra are less complex and esier to interpret than the spectra produced by secondary-ion mass spectrometry, which are complicated by the rapid ion/-molecule reactions that can occur directly above the surface between adsorbates and substrate atoms. In the laser-desorption method, these complications are avoided because the ions are formed after the desorbed molecules have moved away from the surface and have expanded into the vacuum. The sensitivity of the laser-desorption F.t.m.s. method is tested. For carbon monoxide adsorbed on platinum, the detection limit is ca. 5 × 10^?6 monolayer per cm². For naphthalene, a single laser pulse at 248 nm produces abundant molecular ions even when the electron beam is turned off. The ions appear to be produced by resonance-enhanced multiphoton ionization rather than a thermal process. In these experiments, multiphoton ionization of naphthalene at 248 nm is about 35-fold more sensitive than electron ionization.     

Reactions of HBr+ ions in the 2Π i , v + quantum states with H2 and HBr molecules

The mechanisms and kinetics of low-temperature ion-molecular reactions between the Br⁺, HBr⁺, and DBr⁺ions and the HBr, DBr, H₂, and D₂ molecules were studied. The HBr⁺ (i,v ⁺) and DBr⁺(i,v ⁺) ions were prepared in separate spin-orbit (i) and vibrational (v ⁺) states by resonance multiphoton ionization in a free flow of halogen halides (HBr and/or DBr) with hydrogen, deuterium, or inert gases (Ar, He). The effectiveness of various reaction channels, including the exchange of charges, H and D atoms, and H⁺andD⁺ ions, was studied. The quantitative data on the kinetics of these reactions were obtained for separate quantum states of the ions. The resonance ionization of one of the two ion isotopomers H⁷⁹Br⁺(D⁷⁹Br⁺) or H⁸¹Br⁺(D⁸¹Br⁺) was used to study and compare the effectiveness of various ion-molecular reaction channels.     

Photodissociation Dynamics of 2-Iodotoluene Investigated by Femtosecond Time-Resolved Mass Spectrometry

The photodissociation dynamics of 2-iodotoluene following excitation at 266 nm have been investigated employing femtosecond time-resolved mass spectrometry. The photofragments are detected by multiphoton ionization using an intense laser field centered at 800 nm. A dissociation time of 38±50 fs was measured from the rising time of the co-fragments of toluene radical (C₇H₇) and iodine atom (I), which is attributed to the averaged time needed for the C-I bond breaking for the simultaneously excited nσ^* and ππ^* states by 266 nm pump light. In addition, a probe light centered at 298.23 nm corresponding to resonance wavelength of ground-state iodine atom is used to selectively ionize ground-state iodine atoms generated from the dissociation of initially populated nσ^* and ππ^* states. And a rise time of 40±50 fs is extracted from the fitting of time-dependent I⁺ transient, which is in agreement with the dissociation time obtained by multiphoton ionization with 800 nm, suggesting that the main dissociative products are ground-state iodine atoms.     

Resonant laser mass spectrometry: fragmentation patterns from 13C1 naturally labeled molecules

The potential of resonance-enhanced multiphoton ionization (REMPI) mass spectrometry to pick out the fragmentation pattern due to ¹³C₁-isotopomers from the fragmentation pattern due to the unlabeled molecule, in non-isotope-enriched samples, has been explored. Toluene, n-propylbenzene, ortho-diethylbenzene, and tert-butylbenzene have been used as testing samples. The fragmentation patterns of the unlabeled molecule and of the natural abundance ¹³C₁-isotopomer have been measured in a time-of-flight mass analyzer by exciting successively the S₁ ← S₀ origins of the ¹²C-monoisotopic molecule and ¹³C₁-isotopomers. Fragmentation mechanisms are not completely clear from the comparison of these mass spectra, but the method can be applied to low concentration enriched compounds labeled in known positions.     

Photoionization of NO molecule in two-color femtosecond pulse laser fields

A theoretical approach used for studying the multiphoton ionization of NO molecule in two-color strong femtosecond pulse laser fields is proposed. Time- and energy-resolved photoelectron energy spectra are calculated for describing the photionization process. The NO molecule is first excited to the D²Σ⁺ Rydberg state. The pump and probe pulses then couple the D²Σ⁺ with M²Σ⁺ states via a two-photon Raman coupling and a laser-induced continuum structure state. Three channels from the D²Σ⁺ and M²Σ⁺ states to ionization state are described. The population transfers through the continuum state and the Raman coupling passage are also calculated.     

Two-color resonant multiphoton ionization study of CO. The rotational energy transfer behavior of CO A1 Πstate

Two-color resonant multiphoton ionization provides a novel technique, complementary to LIF, for studying state-to-state molecular relaxation dynamics. A set of formulae applicable to two-color pulsed laser ionization experiments has been derived to evaluate the rotational energy transfer probability for CO(A)-CO(X)collision pairs. The magnitude of this probability is on the order of 1/10 for rotational quantum jump ΔJ = ±1 and diminishes with increasing ΔJ. For the same > J(<±3), a transition with e/f symmetry conservation is always more probable than that with an e/f change.     

Fragmentation patterns in multiphoton ionization: a statistical interpretation

The simplest statistical theory wherein the mass spectral fragmentation pattern is governed by the mean energy is derived and compared with the observed results for resonance-enhanced multiphoton ionization of benzene. By varying the mean energy absorbed it is possible to span the range of different patterns up to the dominance of C⁺₁ions above 37 eV/molecule.     

Multiphoton ionization of magnesium and calcium atoms by short and intense laser pulses

Single and double ionization of magnesium and calcium atoms following Nd: YAG laser multiphoton excitation at 1064 and 532 nm have been studied by employing pulses of 35 ps and 200 ps duration at intensities of the order of 10¹⁰–2×10¹³ W/cm². The dependence of ion formation on the laser intensity has been measured and the slopes of the linear parts of the log-log plots and the ratios of saturation intensities for two pulse durations have been compared with the predictions of the scaling law. No evidence for a pure direct double ionization process has been obtained.