Theory of Multi-Photon Ionization of Atoms and Molecules

In this paper we present a density matrix formalism to treat one-photon ionization and multiphoton ionization of atoms and molecules in the presence of an external field. It is shown that Fano's results for one-photon ionization without an external field can easily be obtained from our formulation. For comparison, the Green's function method applied to multiphoton ionization of molecules is also presented.     

Communication: Conditions for one-photon coherent phase control in isolated and open quantum systems

Coherent control of observables using the phase properties of weak light that induces one-photon transitions is considered. Measurable properties are shown to be categorizable as either class A, where control is not possible, or class B, where control is possible. Using formal arguments, we show that phase control in open systems can be environmentally assisted.     

Coherent control of indirect photofragmentation in the weak-field limit: control of transient fragment distributions

We demonstrate theoretically that laser-induced coherent quantum interference control of asymptotic states of dissociating molecules is possible--even in the (one-photon) weak-field limit starting from a single vibrational eigenstate--when resonances are in play. This is illustrated for the NaI molecule, where it is shown that the probability of observing atomic fragments as well as the distribution of their relative momenta can be changed by a phase modulated pulse with a fixed bandwidth. This type of control is restricted to finite times during the indirect fragmentation.     

Vibrational spectroscopy of the pyridazine cation in the ground state

Vibrational structure of the pyridazine cation in the ground state has been revealed by a vacuum-ultraviolet mass-analyzed threshold ionization (VUV-MATI) spectroscopy. The adiabatic ionization energy is precisely measured to be 70241 +/- 6 cm(-1) (8.7088 +/- 0.0007 eV). The origin is very weakly observed, while a long progression of the nu9(+) (a1) band of which the fundamental vibrational frequency is 647 cm(-1) is predominantly observed. The nu9(+) (a1) mode progression combined with one quantum of the nu3(+) (a1) band at 1698 cm(-1) is found to be even stronger. Many other weakly observed vibrational features of the pyridazine cation are identified in the vibrational energy of 0-3500 cm(-1). The structural change of pyridazine upon ionization, reflected in the vibrational spectrum obtained by the one-photon direct ionization process, is theoretically predicted by ab initio calculations. Ring distortion including contraction of the N=N bond should be responsible for strong excitations of nu3(+) and nu9(+) modes. Franck-Condon analysis is given for the comparison of the experiment and theory.     

242-260nm波长区CS2分子的多光子解离电离

在242－260nm波氏范围通过CS2分子的共振增强多光子电离（REMPI）获得了母体离子CS和碎片离子的分质量激发谱．在λ＜246.4nm区间，CS激发谱上呈现出来源于CS2双光子电离的弥散谱带,碎片离子激发谱的归属强烈提示多光子过程中有中性基电子态的CS和S（经由CS2的光解离）产生：（1）CS 的谱带主要来源于中性CS碎片经由单光子跃迁产生的（1＋1）共振增强电离，（2）除了部分S 的谱峰来自CS 的光解外，多数S 的锐谱峰来自中性S原子经由3p3（2D0）4p，3p3（4S0）np（n＝6，7，8）←3p43pJ（J＝2，1,0）双光子跃迁产生的（2＋1）共振增强电离．     

Transition probabilities for coherent multiphoton absorption processes

The feasibility of coherent multiphoton propagation effects such as two-photon self-induced transparency is examined by calculating the coherent transition probabilities for a multiphoton process. For a two-photon excitation of a three-level system, periodic probability functions are obtained which increase smoothly as the intermediate state approaches resonance with the radiation field. The results for a coherent multiphoton excitation of a multilevel system are an extension of Rabi's “strong signal theory” for a one-photon excitation of a two-level system.     

Rotational state selection of a CH3I+ ion beam using vacuum ultraviolet-mass-analyzed threshold ionization spectroscopy: characterization using photodissociation spectroscopy

The A(2)A(1)<--X(2)E(3/2) transition of CH(3)I(+) was investigated by photodissociation (PD) of the cation generated by one-photon mass-analyzed threshold ionization (MATI). Compared to the PD spectrum obtained by excitation of the cation in the main 0-0 band in the MATI spectrum, those obtained by excitation of the cations in the satellite structures showed substantially simplified rotational structures for nondegenerate vibronic bands. Spectral simplification occurred because each satellite consisted mostly of cations with one K quantum number. Spectroscopic constants in the ground vibronic state and in the 2(1)3(5), 2(1)3(8), 3(9), and 3(13) nondegenerate vibrational states in A(2)A(1) were determined via spectral fitting. Also, those in the 2(1)3(n)6(1) (n=1?) degenerate state, which had been reported previously, was improved. The K quantum number in each satellite determined by the present high resolution study was compatible with the prediction by the symmetry selection rule for photoionization. That is, the K quantum number of the ion core in high Rydberg states accessed by one-photon excitation was found to be conserved upon pulsed field ionization. This work demonstrates generation of mass-selected, vibronically selected, and K-selected ion beam by one-photon MATI.     

Mode specific photodissociation of CS2(+) via the A2Π(u) state: a time-sliced velocity map imaging study

The vibrationally mediated photodissociation of CS(2)(+) cations via the A(2)Π(u)(ν(1),ν(2),0) state has been studied by means of the velocity map ion imaging technique. The measurements were made with a double resonance strategy. The CS(2)(+) cations were prepared by a (3 + 1) resonance enhanced multiphoton ionization method. The photo-fragment excitation spectrum of S(+) was recorded by scanning the photolysis laser via the A(2)Π(u)(ν(1),ν(2),0) state. By fixing the photolysis laser wavelength at the specific vibrational state, the (1 + 1) photodissociation images of S(+) photofragments from numerous vibrationally mediated states have been accumulated. The translational energy release spectra derived from the resulting images imply that the co-fragments, CS radicals, are both vibrationally and rotationally excited. The one-photon photodissociation without the vibrational state selection has also been performed. Comparing the vibrationally mediated photodissociation with one-photon photodissociation observations, clear evidence of vibrational state control of the photodissociation process is observed.     

243～263nm CH3COCH3 的多光子电离研究

在243～263 nm紫外光波段通过质量选择光电离激发谱研究了丙酮（CH3COCH3）的光化学反应通道。分析母体离子CH3COCH3+和碎片离子CH3CO+ 、 CH3+的光电离激发谱和质谱峰宽可以知道: 此光波段丙酮分子的光化学反应主要包括了丙酮分子经由(S1,T1)中间态产生母体离子的（1+1）双光子电离通道,母体离子进一步解离产生碎片离子CH3+的“光电离-光解离”通道和丙酮分子经由(S1,T1)中间态解离成中性自由基碎片CH3CO后再进一步被双光子电离的“光解离-光电离”通道。由母体离子光电离激发谱双光子阈值波长（255.67 nm）给出的丙酮电离势（IP）为（9.696±0.004）eV。     

Experimental and theoretical investigations of isomerization reactions of ionized acetone and its dimer

Ionization dynamics of acetone and its dimer in supersonic jets is investigated by a combination of experimental and theoretical techniques, both of which have recently been developed. In experiments, the neutral and the cationic structures are explored by infrared predissociation spectroscopy with the vacuum-ultraviolet photoionization detection schemes. Reaction paths following the one-photon ionization of the acetone monomer and its dimer have been studied by the joint use of several theoretical methods including the ab initio molecular dynamics, the global reaction route mapping, the intrinsic reaction coordinate, and the artificial force induced reaction calculations. Upon one-photon ionization, the dimer isomerizes to the H-bonded form, in which the enol cation of acetone is bound to the neutral molecule, while this enolization is energetically forbidden in the acetone monomer. The enolization of the dimer cation occurs through a two-step proton-transfer from the methyl group of the ionized moiety, and is catalyzed by the neutral moiety within the dimer cation.     

One- and two-photon spectra of some selected molecules: A comparative study

The complementarity of one- and two-photon spectroscopy has been utilised for throwing light on the following problems of chemical interest: (1) Weak interaction between identical chromophores separated by insulating bridges gives rise to split states of different symmetries. Two-photon spectroscopy (TPA), in conjunction with one-photon absorption (OPA), has been used to identify the states and hence to estimate the magnitude of interaction in bimolecules and trimolecules. From the shifts between the one- and the two-photon spectra, the splittings have been estimated. Calculations confirm that the dominant interaction is the through-bond one. (2) The second type of problem is the identification ofg andu vibrations in molecules. We have initiated studies on three molecules in jet-cooled conditions: 9,10-dihydro-anthracene (DHA). 9,10-dihydro-phenanthrene (DHP) and octa-fluoronaph-thalene (OFN). Only the one-photon fluorescence excitation spectra have so far been obtained by us and the TPA spectra are under investigation. (3) The third class of molecules discussed here are the Ln³⁺ complexes wheref ⁿ⇒ fⁿ transitions are intrinsically two-photon allowed. We have studied two GD³⁺ single crystals. The CF-splittings, observed clearly in TPA, have been fitted with a parametric model. Some of our observations on the variations of TPA intensity patterns from crystal to crystal, such as circular:linear polarisation ratios, relative intensities of transitions to differentJ-states, do not quite fit in with the Axe-Judd-Downer model. The discrepancies call for a reappraisal of the role of ligand in the TPA process.     

One-photon ionization spectroscopy of jet-cooled oxazole and thiazole: the role of oxygen and sulfur in the pi-conjugation of heterocyclic compounds

One-photon mass-analyzed threshold ionization (MATI) spectroscopy of jet-cooled oxazole and thiazole has been carried out to give the precise adiabatic ionization energies of 9.5959+/-0.0006 and 9.3633+/-0.0009 eV, respectively. The structural change upon ionization has been revealed in the vibrationally resolved one-photon MATI spectra. Simulations based on the Franck-Condon analysis using the molecular structures calculated by the density functional theory reproduce the experiment very well for both molecules. The ionization-driven structural change of thiazole is quite different from that of oxazole in terms of the detailed geometrical shape, ascribed to the difference in the pi-conjugation nature of two molecules. The role of oxygen and sulfur in the stabilization of heterocyclic systems is discussed through the inspection of the calculated molecular orbitals involved in the photoionization.     

Difficulties in the computation of physical observables due to a truncated CI

It is proved that the values of various types of physical observables issued from a truncated CI are not size consistent. These observables include electronic transitions, ionization potential, and all the quantities depending on the one-electron density matrix γ. Numerical examples are given. It is found that the error due to this size inconsistency can be important for systems of moderate size (e.g., 16% on a matrix element of γ for a 20-electron system).     

One- and two-photon ionization of DNA single and double helices studied by laser flash photolysis at 266 nm

The ionization of the DNA single and double helices (dA)20, (dT)20, (dAdT)10(dAdT)10 and (dA)20(dT)20, induced by nanosecond pulses at 266 nm, is studied by time-resolved absorption spectroscopy. The variation of the hydrated electron concentration with the absorbed laser intensity shows that, in addition to two-photon ionization, one-photon ionization takes place for (dAdT)10(dAdT)10, (dA)20(dT)20 and (dA)20 but not for (dT)20. The spectra of all adenine-containing oligomers at the microsecond time-scale correspond to the adenine deprotonated radical formed in concentrations comparable to that of the hydrated electron. The quantum yield for one-photon ionization of the oligomers (ca. 10(-3)) is higher by at least 1 order of magnitude than that of dAMP, showing clearly that organization of the bases in single and double helices leads to an important lowering of the ionization potential. The propensity of (dAdT)10(dAdT)10, containing alternating adenine-thymine sequences, to undergo one-photon ionization is lower than that of (dA)20(dT)20 and (dA)20, containing adenine runs. Pairing of the (dA)20 with the complementary strand leads to a decrease of quantum yield for one photon ionization by about a factor of 2.     

Nonlinear intensity dependence of the rate coefficient in unimolecular reactions induced by monochromatic infrared radiation

It is shown that essentially incoherent, stepwise one-photon transitions arising in dense multilevel structures of polyatomic molecules irradiated with monochromatic coherent infrared radiation can give rise (under specified conditions) to a more than proportional intensity dependence of the steady-state rate coefficient in unimolecular reactions induced by monochromatic infrared radiation.     

Coherent and incoherent orientation and alignment of ICN photoproducts

We report extended measurements of the rotational polarization and correlated angular distribution of CN photofragments from ICN photodissociation, with a particular emphasis on the creation and detection of molecular orientation with circularly-polarized light. Doppler profiles of the nascent photoproducts are measured by Frequency-Modulated (FM) transient absorption, and the resulting high signal-to-noise data are valuable for verifying the form of the angular correlations between the recoil velocity, the photofragment rotational angular momentum, and the space-fixed frame defined by the dissociation polarization. A space-fixed bipolar moment notation can be used for an unambiguous characterization of the maximal set of polarization properties that can be created with one-photon excitation and detected with one-photon Doppler-resolved absorption spectroscopy. Relating the observed polarization moments to the various coherent and incoherent, adiabatic and non-adiabatic mechanisms, that have been derived and verified extensively in the case of diatomic photodissociation to polarized atomic fragments, is not unambiguous in the case of diatomic fragments from triatomic precursors. Constraints among various polarization moments confirmed in the case of diatomic dissociation are not confirmed in this triatomic case, where the perpendicular transitions to non-degenerate A' and A' components of a linear Omega = 1 state are qualitatively different from excitation to degenerate Omega = +/-1 states in a diatomic molecule.     

Two-photon excitation studies of hypocrellins for photodynamic therapy

The photophysical and photochemical properties of hypocrellins (HA and HB) are examined with two-photon excitations at 800 nm using femtosecond pulses from a Ti:sapphire laser. The two-photon excited fluorescence spectra of HA and HB are very similar to those obtained by one-photon excitation, which may indicate that the two-photon induced photodynamic processes of hypocrellins are similar to one-photon induced photodynamic processes. The two-photon excitation cross sections of HA and HB are measured at 800 nm as about 34.8 x 10(-50) cm(4) s/photon and 21.3 x 10(-50) cm(4) s/photon, respectively. The large two-photon cross sections of both HA and HB, suggest that the hypocrellins can be potential two-photon phototherapeutic agents. As an example for two-photon photodynamic therapy of hypocrellins, we also further examine the cell-damaging effects of HA upon two-photon illumination. Our preliminary results of cell viability test indicate hypocrellins can effectively damage the Hela cells under two-photon illumination.     

On Metal-Atom Clusters IV. Photoionization thresholds and multiphoton ionization spectra of alkali-metal molecules

The investigation of electronic and structural parameters of metal molecules as a function of size may be decisive for understanding and control of heterogeneous catalysis with finely divided metals. Metal-atom clusters can be prepared by several methods, most of which yield a molecular mixture only. Expansion of an atomic vapour into vacuum through a supersonic nozzle creates a complex cluster-spectrum which has been investigated by photoionization using a mass-spectrometer as selective detector. Broad band single photoionization ( PI. ) yields the abundances and ionization thresholds for Na_x(x?16), K_x(x?12) and Na_xK_y(x+y?6). Rough indications about other deactivation channels of excited alkali-molecules are obtained from an analysis of the photoionization efficiency (PIE.) curves. Two-photon PIE. curves with narrow-band (laser) light sources give an accurate photoionization threshold value and detailed information on ionization processes. Two-photon ionization spectra via a real intermediate state reached by laser excitation are equivalent to normal absorption spectra, if the exciteation step is controlled by the true transition probabilities. By investigation of the power dependence of the ion current of Na as a function of the wavelength of the tunable cw dye laser (excitation) and of the ionizing Ar⁺- or Kr⁺-laser, conditions under which true spectra are obtained have been clarified, in good agreement with a photon-kinetic model of the processes involved. Vibronic and rovibronic spectra of several transitions in Na₂ and K₂ have been measured. Similarly a spectrum of Na₃ has been determined mass-selectively. The scope of the new method for an absorption spectroscopy in molecular beams is discussed.     

Comparative studies of the behavior of neutral and ionic ferrocene derivatives under different conditions of electrospray ionization

We have studied the behavior of ferrocene CpFeCp (FcH), ferrocenium triiodide [FcH]⁺I₃⁻, dimethylaminomethylferrocene FcCH₂NMe₂ and its trimethylammonium salt [FcCH₂NMe₃]⁺I⁻ under the conventional conditions of electrospray ionization (ESI), when the substance solution is subjected to spraying, and in two versions of desorption electrospray ionization (DESI), when the sprayed solvent bombards the surface of solid or liquid samples. In addition to these techniques, the behavior of neutral compounds under conditions of electrospray ionization of vapors of the studied compounds in a gas phase (ESI_V) has been investigated. It has been shown using the examples of ferrocene and its dimethylaminomethyl derivative that the detection limits for these compounds occurring in a gas phase are comparable within an order of magnitude with their detection limits under the ESI and DESI conditions of solid and liquid samples. The high effectiveness of ionization of analyte vapors makes it possible to use the ESI method not only in combination with liquid (conventional ESI technology) and thin layer chromatography (DESI), but also with gas liquid chromatography (ESI_V). Thus, the electrospray ionization becomes a universal method allowing studies of a compound under the natural conditions in any state of aggregation, that is, solid, liquid, and gas. With the help of statistical methods for designing experiments (complete factorial experiment), quantitative evaluation of the influence of experimental parameters on the ion-formation processes under different ESI conditions has been carried out, which makes it possible to purposefully select the optimal conditions to record the ESI mass spectra with a minimum number of experiments. Moreover, analysis of the dependences of the mass spectra on the experimental parameters can serve as an instrument for studying the details of the ion-formation mechanisms depending upon different ways of ionization.     

Laser mass spectrometry with circularly polarized light: two-photon circular dichroism

Extensive studies of two-photon circular dichroism (TPCD) on 3-methylcyclopentanone are presented following the first TPCD experiment of gas phase molecules by Compton et al. [J. Chem. Phys.125 (2006) 144304]. (2 + 1)-Multiphoton ionization in a specially designed time-of-flight mass analyzer has been used to perform these studies. CD of two-photon transitions from the molecular ground state to low lying Rydberg states is strongly enhanced with respect to corresponding one-photon transitions in good agreement with Compton. Differences between CD values determined via the molecular ion and via fragment ions indicate strong molecular ion CD effects. This would be the first time that circular dichroism of isolated molecular ions has been measured.