Laser multiphoton ionization of aromatic molecules in nonpolar liquids

The laser multiphoton ionization (MPI) of fluoranthene in tetramethylsilane (TMS) and of azulene in n-tridecane, n-pentane, 2,2,4,4-tetramethylpentane, TMS and tetramethyltin is reported. Three distinct types of MPI mechanisms have been identified: two-photon ionization, stepwise three-photon ionization and mixed two- and three-photon ionization. The stepwise three-photon process consists of two-photon excitation, relaxation to a lower lying excited state with a lifetime comparable to the laser pulse duration (for azulene this state is the S₂ while for fluoranthene both the S₁ and S₂ states) and subsequent ionization with the absorption of a third photon. The ionization threshold of azulene in each liquid has been determined and found to vary linearly with the V₀ of the liquid.     

Investigation of laser induced structures in multiphoton ionization of magnesium

We examine the effect of the excitation of an autoionizing state on electron spectra resulting from multiphoton-ionization. We study the case of above threshold three-photon ionization of magnesium in the region of the 3p3d ¹ P ⁰ resonance, with conditions similar to a two-colour experiment recently performed. The electron spectrum is compared to the partial spectrum corresponding to the region located just above the threshold (which is populated through two-photon absorption from a probing laser). The calculations confirm the experimental observations, a laser induced continuum structure (LICS) is observed in this partial spectrum. The interference process in examined in detail and a Fano-like parametrization of the two-photon ionization electron profile is proposed.     

Multiphoton dissociation/ionisation of dimethyl sulphide (CH3SCH3) at 355 and 532 nm

Multiphoton dissociation/ionization has been studied for CH₃SCH₃ at 355 and 532 nm using a time-of-flight mass spectrometer. The major ion signals observed at 355 nm are C⁺, CH₃ ⁺, HCS⁺, CH₂S⁺, CH₃S⁺ and CH₃SCH₃ ⁺. Power dependence studies at 355 nm show a (2+1) REMPI process for the formation of parent ion. Peaks atm/e = 46, 47 and 61 show two-photon laser power dependence whereasm/e = 15 and 45 peaks show four-photon dependence. However, in 532 nm photo-ionisation, no parent ion signal is observed. A peak atm/e = 35 corresponding to SH₃ ⁺ has been observed. SH₃ ⁺ has been suggested to originate from CH₃SCH₂ ⁺ via a cyclic transition state. Photoionisation results of CH₃SCH₃ have been compared with those of CH₃SSCH₃, at these two wavelengths.     

Influence of the vibrational quantum number of the resonant state in resonant multiphoton ionization/dissociation of hydrogen molecules

We have studied the resonant multiphoton ionization of hydrogen in a three-photon excitation, one-photon ionization scheme. Superimposed on the ionization process we find a dissociation mechanism which manifests itself in a strong H⁺ signal. The ratio of H⁺ to H⁺₂ signals depends on the vibrational quantum number v' of the intermediate state and on the laser intensity. We present a simple model which qualitatively reflects this dependence.     

Mechanism of resonant multiphoton ionization dissociation of p-xylene

The mechanism of resonant multiphoton ionization dissociation (RMPID) of p-xylene is investigated theoretically based both on the quantum-mechanical MO calculation of the geometrical structures and dissociation energies of the fragments and on the computation of laser power-dependent mass spectra of the fragments produced by RMPID. The geometries and dissociation energies are calculated by using both the MNDO method and the ab initio method with the 6-31G basis set. The computation of the mass spectra is carried out in the absorption multiple fragmentation model. It is shown theoretically that the two-independent reaction sequence mechanism proposed by Takenoshita et al. upon measuring the laser power dependence can explain semi-quantitatively the mass spectra of the RMPID. From comparison of the measured mass spectra with the calculated ones it is suggested that the absorption multiple fragmentation model originally based on the product phase space theory can be applied to the RMPID involving transition states in the course of reactions such as the retro-Diels-Alder reaction C₅H⁺₅ → C₃H⁺₃ + C₂H₂ by taking into account the corresponding activation energy instead of the dissociation energy.     

A new eletronic spectrum of the SiH3 radical observed using multiphoton ionization spectroscopy

The silyl radical, SiH₃, was observed between 365 and 410 nm using resonance-enhanced multiphoton ionization spectroscopy. The spectrum arises from two-photon resonance states which lie between 49229 and 54014 cm⁻¹. A vibrational progression interval of ≈ 800 cm⁻¹ was assigned to the resonant intermediate state symmetric deformation mode, ν₂.     

Sub-picosecond laser ionization of free C60 and C70 at 248 nm

Delayed ionization is found to be absent for sub-picosecond laser excitation of free C₆₀ and C₇₀ at 248 nm. The autocorrelation trace obtained for C ₆₀ ⁺ in a laser time-of-flight (TOF) mass spectrometer using two time-delayed and collinear 248 nm ultrashort laser pulses has a width of 1.1 ps (715 fs for sech² pulses), in agreement with the laser pulse duration measurement in NO gas. Both above observations can be explained by direct ionization of C₆₀ via coherent two-photon absorption by the high intensity sub-picosecond 248 nm laser excitation avoiding the channel leading to delayed ionization.     

Laser induced stepwise and two-photon ionization studies of strontium in the air acetylene flame

Using strontium atoms present as a trace constituent in an air-acetylene flame as an example, a rich laser enhanced ionization spectrum was obtained. One pulsed tunable dye laser was tuned to a transition originating from the ground state (460.73 nm) and another scanned over different spectral regions. The lines obtained were spectroscopically characterized as to the type of absorption process, which included non-resonant processes as well as single wavelength, two-wavelength, and two-photon resonant processes.With a maximum irradiance of 100 MW cm⁻², two-photon transitions resulting in collisionally assisted ionization included the 5sns and 5dnd Rydberg series (up to 37s and 15d) together with a strong auto-ionizing transition at 431.10 nm. The complexity of the observed ionization spectrum when the irradiance is high indicates that spectral interferences need to be carefully considered in analytical applications.     

Isotope fractionation in two-step photoionization of Li2

A cw argon-ion laser beam intersects a thermal-energy lithium-beam. Sequential two-photon ionization of Li₂ is observed. Li⁺₂ ions are formed by most of the available laser lines. Large isotope fractionations result from this process.     

Laser-induced dissociation of phenetole ions prepared by resonance-enhanced two-photon,two-color ionization

Laser-induced dissociation of phenetole ions has been carried out. The ions have been prepared in a well-defined excited state by resonance-enhanced two-photon, two-color ionization (R2PI/2C). Appearance energies of 2.431 and 2.82 eV have been determined for the first dissociation pathways leading to C₆H₆O⁺ and C₅H⁶⁺ ionic fragment. The dissociation spectrum as well as detailed dissociation rate constants versus internal energy have been obtained. The dissociation rates are compared with those expected from RRKM theory. A complex reaction scheme has been assumed in order to explain the observed discrepancy.     

Two-photon MPI spectroscopy of alkyl iodides

The two-photon resonant multiphoton ionization (MPI) spectra of methyl iodide, methyl iodide-d₃, ethyl, propyl, and butyl iodide are reported in the 49 000-55 000 cm^?1 region. Four separate transitions to excited states labeled Δ, Π, Σ, Π in increasing energy are expected in this range which result from the excitation of an iodine 5pπ electron to the 6s molecular Rydberg orbital. Two-photon spectroscopy with its different selection rules and unique dependence on the laser polarization is shown to significantly advance the understanding of these transitions. In particular, laser polarization studies identify a state which is strongly two-photon allowed but absent in the UV absorption spectrum as the Σ state. Rotational contours indicate a large geometry change takes place in this transition. The two Π states appear strongly in both the one-and two-photon spectrum. Polarization analysis confirms their electronic symmetry assignment in addition to distinguishing vibronic bands arising from nontotally symmetric vibrations. No evidence is found for the Δ state in the multiphoton ionization spectrum, due to either a small two-photon cross section or a low probability of ionization following the initial two-photon transition. Further complications and characteristics of single laser MPI spectroscopy in the study of two-photon absorption in methyl iodide and other fundamental molecules are discussed.     

Use of multiphoton ionization of molecules for the determination of neptunium traces in solution

The possibility of detecting trace amounts of neptunium atoms and molecular compounds by laser resonance ionization spectroscopy has been investigated in acetylene-air and acetylene-nitrous oxide flames. Experimentally evaluated atomization coefficients of neptunium in the process of thermal dissociation in flames (10^–4% for C₂H₂-air and 10^–2% for C₂H₂–N₂O) do not allow to use atomic ionization signal for determination of neptunium microamounts in solutions. High yield multiphoton ionization of NpO molecule has been realized in fuel rich C₂H₂–N₂O flame. This pehnomenon has been used for determination of trace quantities of Np in solution. The detection limit was 2·10^–9 g/ml.     

Two-photon ionization spectrum of the 1Lb ← S0 transition in toluene

The multiphoton ionization (MPI) spectrum of toluene arising from the ¹B₂ (¹L_b) valence state has been investigated. The state participates as a two-photon resonance. A total of nine excited state fundamentals have been characterized, including three non-totally symmetric vibrations. The toluene MPI spectrum shows a strong resemblance to the two-photon fluorescence excitation spectrum with the strongest transitions taking place to the origin and excited state modes ν₁(a₁), ν₁₂(a₁) and ν₁₄(b)₂). The intensities of the observed fundamentals are rationalized in terms of Franck-Condon and vibronic coupling effects. A major conclusion is, that the primary mechanism for the activity of ν₁₂ is vibronic coupling.     

Experimental and Theoretical Investigation of the 3sp(d) Rydberg States of Fenchone by Polarized Laser Resonance-Enhanced-Multiphoton-Ionization and Fourier Transform VUV Absorption Spectroscopy

The VUV absorption spectrum of fenchone is re-examined using synchrotron radiation Fourier transform spectrometry, revealing new vibrational structure. Picosecond laser (2+1) resonance enhanced multiphoton ionization (REMPI) spectroscopy complements this, providing an alternative view of the 3spd Rydberg excitation region. These spectra display broadly similar appearance, with minor differences that are largely explained by referring to calculated one- and two-photon electronic excitation cross-sections. Both show good agreement with Franck-Condon simulations of the relevant vibrational structures. Parent ion REMPI ionization yields with both femtosecond and picosecond excitation laser pulses are studied as a function of laser polarization and intensity, the latter providing insight into the relative two-photon excitation and one-photon ionization rates. The experimental circular-linear dichroism observed in the parent ion yields varies strongly between the 3s and 3p Rydberg states, in good overall agreement with the calculated two-photon excitation circular-linear dichroism, while corroborating other evidence that the 3p_z sub-state plays no more than a very minor role in the (2+1) REMPI spectrum. Vibrationally resolved photoelectron spectra are recorded with picosecond pulse duration (2+1) REMPI at selected intermediate vibrational excitations. The 3s intermediate state displays a very strong Δv=0 propensity on ionization, but the 3p intermediate evidences more complex vibronic dynamics, and we infer some 3p→3s internal conversion prior to ionization.     

Infrared multiphoton dissociation of CHFCl2

We have studied the dependence of the IR laser induced multiphoton dissociation of CHFCl₂ on laser wavelength and fluence, number of laser pulses and initial CHFCl₂ pressure. Multiphoton dissociation spectrum presents a broad maximum centered at 9.342 μm. Dissociation has a strongly dependence on initial CHFCl₂ pressure and laser fluence. There exists a pressure lower limit (≈ 0.4 mb) below which no significant dissociation occurs. Up to 2.1 J/cm² no unimolecular dissociation takes place in a significant proportion. The only radical we have been able to clearly detect by real time optical absorption technique is CFCl. This supports the mechanism reaction CHFCl₂ + nhν → CFCl + HCl.     

The production of Te atoms by the multiphoton dissociation of diethyltelluride

Detection of Te atoms in the ground ³P^2,1,0 states has been accomplished using two-photon laser-induced fluorescence (LIF) and “2 + 1” multiphoton ionization (MPI). Te atoms were produced by multiphoton dissociation of (C₂H₅)₂Te in the region 358–395 nm. The LIF and MPI experiments utilize selective excitation of the 6p ³P_J′←5p ³P_J″ and 7p ³P_J′←5p ³P₂ transitions by two-photon absorption. Line strength intensities for the individual J′ ← J″ fine structure pairs of the 6p ³P_J′ ← 5p ³P_J″ two-photon transitions were calculated and compared to the LIF data. Intensities of these transitions of Te atoms are compared to analogous ³P ← ³P transitions in S and O atoms.     

Detection of nitrogen rotational distributions by resonant 2 + 2 multiphoton ionization through the a1Πg state

Characterization of laser 2 + 2 multiphoton ionization of nitrogen to obtain rotational state distributions has been investigated via the resonant two-photon transition a ¹Π_g(ν = 1) ← X ¹Σ_g(ν = 0). For room-temperature nitrogen, the spectral intensities and state distribution are directly related and give rotational temperatures of 290 ± 20 K. For power densities of 3 GW/cm², the ionization probability is 1 × 10⁻⁵ per N₂ molecule per average rotational state.     

Quasienergy formalism for intense field multiphoton ionization of atoms induced by circularly polarized radiation

A practical non-perturbative approach is presented for multiphoton ionization of atoms induced by circularly polarized radiation. By use of co-ordinate rotation transformation and L²-discretization of the atomic continuum, the complex energy spectrum of a stationary quesienergy operator can be located and multiphoton ionization rates determined as a function of time and arbitrary photon intensity. The theory is applied to the two-photon ionization of the H atom in intense fields.     

Sequence-specific fragmentation of benzyloxycarbonyl dipeptides by resonance-enhanced multiphoton ionization

The mechanism of fragment ion formation in resonance-enhanced multiphoton ionization (REMPI) of benzyloxycarbonyl (CBZ)-derivitized dipeptides is presented. At 266 nm, the entire multiphoton process can be thought of as a two-part scheme where ionization occurs by resonant two-photon ionization followed by photodissociation of the created ions. When the energy of two photons exceeds the molecular ionization energy by a significant amount, REMPI has the advantage of producing both parent ions and low appearance energy fragments in large amounts. For CBZ dipeptides, resonant two-photon ionization at 266 nm produces parent ions as well as A type sequence ions with high abundance. On the other hand, a three-photon process (resonant two-photon ionization followed by parent ion photodissociation) forms sequence-related ions which also involve complex fragmentations of the CBZ chromophore. These results are compared to mass spectra obtained by other ionization/dissociation methods and to REMPI mass spectra of related compounds. Factors related to molecular structure elucidation based upon REMPI mass spectra are discussed. Enhanced isomer distinction is demonstrated for CBZ-leu-ala-OCH₃ and CBZ-ile-ala-OCH₃ based upon REMPI fragmentation.     

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.