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A laser based vacuum ultraviolet (VUV) light source using resonance enhanced four wave difference mixing in xenon gas was developed for near threshold ionization of organics in atmospheric aerosol particles. The source delivers high intensity pulses of VUV light (in the range of 1010 to 1013 photons/pulse depending on wavelength, 5 ns FWHM) with a continuously tunable wavelength from 122 nm (10.2 eV) to 168 nm (7.4 eV). The setup allows for tight (

On metals such as Zr, during hydrogen exposure, dissolution competes with desorption; this competition can be probed by thermal desorption at different heating rates. In the case of desorption from preadsorbed hydrogen, only ∼1% of the hydrogen can be desorbed even at heating rates of >10 10 K s −1 . Recent measurements of the dynamics of hydrogen released by water dissociation on Zr(0 0 0 1) [G. Bussiere, M. Musa, P.R. Norton, K. Griffiths, A.G. Brolo, J.W. Hepburn, J. Chem. Phys. 124 (2006) 124704] have shown that the desorbing hydrogen originates from the recombinative desorption of adsorbed H-atoms and that over 25% of the water collisions lead to hydrogen desorption. To gain further insight into the desorption and dissolution of hydrogen and in an attempt to resolve the paradox of the different desorption yields from H 2 vs. H 2 O exposures, we report new measurements of the laser induced thermal desorption (LITD) of hydrogen from Zr(0 0 0 1) at initial temperatures down to 90 K. The low temperature was chosen because work function measurements suggested that hydrogen adsorbed into only the outermost (surface site) of the two available adsorption sites (surface and subsurface), from which we postulated much more efficient desorption at high heating rates compared to desorption from the sub-surface sites. However, hydrogen desorption by LITD from Zr(0 0 0 1) at 90 K still only accounts for 1% of the adsorbed species, the remainder dissolving into the bulk at LITD heating rates. The different yields alluded to above remain unexplained (Bussiere, 2006).

▪ Abstract Ion pair dissociation processes may be studied using coherent vacuum ultraviolet laser sources in a manner entirely analogous to photoelectron spectroscopy, albeit with the anion playing the role of a heavy electron. If the excitation energy is above the dissociation energy and the kinetic energy of the fragment is measured using ion imaging, this approach is termed ion pair imaging spectroscopy (IPIS) and is related to conventional photoelectron spectroscopy. If the excitation energy is just below the dissociation energy and pulsed-field dissociation is employed, this approach is analogous to mass analyzed threshold ionization (MATI) spectroscopy and is termed threshold ion pair production spectroscopy (TIPPS). These approaches provide a novel means of investigating ion thermochemistry and spectroscopy and superexcited state decay dynamics at high resolution.

Preliminary results of a high resolution coherent vacuum ultraviolet study of photoion-pair formation in HF/DF are reported. These results include threshold ion-pair production (TIPPS) and total photoion-pair yield spectra, which show sharp resonances due to predissociating Rydberg states. Detailed analysis of the spectra provides precise energies for the ion-pair thresholds, and insight into the mechanism for ion-pair formation.

Threshold ion-pair production spectroscopy (TIPPS) has been applied to two isotopomers, HCl and DCl. From the high-resolution TIPP spectra the ion-pair thresholds of the two molecules have been precisely measured. Combined with the known ionization energy of H(D) and the electron affinity of Cl, the difference between their bond dissociation energies is calculated, and therefore an experimental determination of the effect of Born–Oppenheimer breakdown on the dissociation limit of the ground state potential curve has been obtained. The difference in De for the two isotopomers was found to be: De(H–Cl)−De(D–Cl)=3.2±1.0 cm−1. The bond energy for HCl was in agreement with our previous determination, D0(H–Cl)=35748.2±0.8 cm−1. These results are compared to a recent study of Born–Oppenheimer breakdown in HCl by Coxon and Hajigeorgiou, where high resolution spectroscopic data was used to fit Born–Oppenheimer breakdown correction terms for the intramolecular potential function. The present study also measured the high resolution spectra for photoion-pair production for HCl and DCl in the threshold region for ion-pair production (∼86 nm). Although there is qualitative agreement between the current results and previous experimental and theoretical work, there are some important differences. The possible mechanism for ion-pair formation in HCl and DCl is discussed in light of these high resolution results.

We present a novel method for recording the absorption spectra of cations of non-volatile molecules using resonance enhanced multiphoton dissociation of trapped ions. Ions are produced through a 2-laser desorption/ionization process in an RF ion trap mass-spectrometer. A third tunable visible laser induces fragmentation of the trapped ions with a yield dependent on the absorption cross-sections of the ions. Using this method we have recorded the visible absorption spectrum of two PAH isomers, phenanthrene and anthracene.

We use an optical centrifuge to excite coherent rotational wave packets in N$_2$O, CS$_2$ and OCS molecules with rotational quantum numbers reaching up to J=465, 690 and 1186, respectively. Time-resolved rotational spectroscopy at such ultra-high levels of rotational excitation can be used as a sensitive tool to probe the molecular potential energy surface at inter-nuclear distances far from their equilibrium values. Significant bond stretching in the centrifuged molecules results in the growing period of the rotational revivals, which are experimentally detected using coherent Raman scattering. We measure the revival period as a function of the centrifuge-induced rotational frequency and compare it with the numerical calculations based on the known Morse-cosine potentials., Comment: 6 pages, 6 figures

Using the technique of an optical centrifuge, we produce rotational wave packets which evolve in time along either classical-like or non-classical trajectories. After releasing O2 and D2 molecules from the centrifuge, we track their field-free rotation by monitoring the molecular angular distribution with velocity map imaging. Due to the dispersion of the created rotational wave packets in oxygen, we observe a gradual transition between "dumbbell"-shaped and "cross"-shaped distributions, both rotating with a classical rotation frequency. In deuterium, a much narrower rotational wave packet is produced and shown to evolve in a truly classical non-dispersing fashion.

Using an optical centrifuge to control molecular rotation in an extremely broad range of angular momenta, we study coherent rotational dynamics of nitrogen molecules in the presence of collisions. We cover the range of rotational quantum numbers between $J=8$ and $J=66$ at room temperature and study a crossover between the adiabatic and nonadiabatic regimes of rotational relaxation, which cannot be easily accessed by thermal means. We demonstrate that the rate of rotational decoherence changes by more than an order of magnitude in this range of $J$ values and show that its dependence on $J$ can be described by a simplified scaling law.

Intracluster ion–molecule reactions have been studied by dissociative photoionization of mixed acetylene–ethane van der Waals clusters in a supersonic molecular beam. The studies were carried out by recording threshold photoelectron–photoion coincidence mass spectra of the clusters at different photon energies between 10.8 and 11.3 eV. Intracluster reactions have been observed in mixed dimer, trimer, pentamer, and hexamer clusters. In the case of the mixed dimer ion [(C 2 H 2 )(C 2 H 6 )] + , two fragment ions result from intramolecular reactions: C 3 H 5 + (CH 3 loss), and C 2 H 4 + (C 2 H 4 loss). This indicates that the dimer ion isomerizes into the 2-butene ion prior to dissociation, as is the case for ethylene dimer ions. Mixed trimer ions also dissociate by CH 3 loss. No intracluster hydride transfer prior to dissociation is observed, although this reaction channel is important in bimolecular C 2 H 2 + + C 2 H 6 collisions. Two very specific intracluster reactions leading to C 2 H 4 loss were observed for pentamer and hexamer mixed cluster ions. A model involving evaporation and intracluster reactions leading to CH 3 and C 2 H 4 loss is proposed to explain this surprising result.

We present the first threshold ion-pair production spectrum of a triatomic molecule. We have recorded the ion-pair yield spectrum and TIPP spectrum for the H2S → H++SH- ion-pair channel using single-photon excitation. From the TIPP spectrum, we have determined the H−SH bond energy (31451 ± 4 cm-1) to unprecedented accuracy and demonstrated the formation of weakly bound H+−SH- (J‘) ion-pair states, with rotational excitation of the SH- anion up to J‘ = 4. The bound nature of these states, and the assigned spectrum that results from their field dissociation suggests that this technique can be applied to many other triatomic and larger polyatomic molecules in the future, leading to energetic, spectroscopic, and dynamical information about these species.

We have demonstrated that the time-of-flight (TOF) selection method for pulsed field ionization (PFI) photoelectron detection [Jarvis et al., Rev. Sci. Instrum. 70, 2615 (1999)] can also be applied for the detection of PFI-photoions (PFI-PIs) using the two-bunch synchrotron radiation at the Advanced Light Source. By employing the supersonic beam technique to lower the translational temperature of the sample gas, we show that background prompt ions formed in direct and spontaneous autoionization processes arrive at the ion detector in a pattern similar to that of the vacuum ultraviolet light bunches. The PFI-PIs formed at dark gaps can be designed to arrive at the detector in between adjacent prompt ion peaks, enabling the gating of the PFI-PI signal with only minor contamination from background prompt ions. This experiment has revealed important considerations for the design of a general TOF selection scheme for PFI-PI detection using synchrotron radiation.

The threshold ion-pair production spectra at the J" = 0 and J" = 1 thresholds of H2 and J" = 0, 1 and 2 thresholds of D2 obtained with single photon excitation are presented. The ion-pair yield spectra of H2 and D2 over these energy ranges demonstrate strong resonant enhancement, parts of which dominate the TIPPS signals, permitting the assignment of the lower states of these resonances. From those thresholds with weak resonant enhancement (the J" = 0 threshold of H2 and the J" = 1 threshold of D2) a very small direct contribution to ion-pair production can be observed. The behaviour of the TIPPS spectra taken with different applied discrimination fields is understood by modeling the field ionization behaviour of a MATI spectrum of H2, containing both the similarly resonantly enhanced v+ = 8 S(0) ionization threshold and the non-resonantly enhanced S(1) ionization threshold. From the H2 J" = 1 and D2 J" = 0 TIPPS spectra the energetic field-free thresholds of the H2 and D2 ion-pair limits were determined to be 139,714.8 +/- 1.0 cm-1 and 140,370.2 cm-1 +/- 1.0 cm-1, respectively.

The zero kinetic energy (ZEKE) photoelectron spectrum of CO has been obtained from 22.3–23.75 eV with resolution of 0.7 meV and shows the structure of the D 2 Π (v+=0–8) and 3 2 Σ + (v+=0–4) bands. The intensity with which these are formed at threshold, band origins, rotational constants and spin orbit splittings of the D 2 Π state are reported. The decreased lifetimes of the D 2 Π , v+=3, 4 levels are also recorded. All states observed are above the lowest dissociation limit of the ion, and these spectra provide information about the intramolecular dynamics in this high energy region.

A recently developed form of threshold ionization spectroscopy has been used to determine the bond energy for HCl to spectroscopic accuracy (±0.8 cm−1). This method is based on excitation to highly vibrationally excited ion-pair states using single-photon transitions from the ground state of HCl. These metastable Rydberg-like states were selectively detected using electricfield induced dissociation. By systematically varying the electric fields involved, and scanning the exciting photon energy, it was possible to determine the field-free energetic threshold for H35Cl+hν→H++35Cl−. Using this energy, together with the known values of the ionization potential of H and electron affinity of Cl, a new estimate for the dissociation energy of HCl was obtained: D0(H35Cl)=35 748.2±0.8 cm−1.

The 000B 2B1–X 2A1 band of CaNH2 has been recorded at high resolution with a laser ablation molecular beam spectrometer. This B 2B1 state is the last of the low-lying electronic states of CaNH2 to be characterized. The pure precession model, which has previously been used to describe the spin-rotation interactions in this family of molecules was used to interpret and correctly assign the spectrum. The detailed analysis of the high resolution spectrum and energy level structure of the B 2B1 state is presented.

As the energy of light incident upon an atom or molecule is increased above that of the lowest ionization energy, a variety of neutral and ionic electronic states can be formed. The former include singly excited states converging onto an ion state with one hole in an inner orbital and doubly excited states converging onto an ionic (satellite) state with two electrons excited from the neutral configuration. These neutral resonances may decay to lower-lying ionic states through the process of autoionization. The ionic states include single hole and satellite ionic states formed directly in conjunction with a free electron. Probing the formation and characteristics of these ionic states is a primary aim of photoelectron spectroscopy. In traditional photoelectron spectroscopy the photon energy is scanned, and all electrons formed with a specific kinetic energy and angular distribution are detected. Threshold photoelectron spectroscopy uses static electric fields to allow the selective detection of electrons with near-zero kinetic energy formed in conjunction with a cationic state. Satellite states can be difficult to probe using this technique; in general, such states are formed at high photon energies, they produce low signal intensities and they contribute to a highly congested spectrum. Further, in order to discriminate against the formation of states through nearby autoionizing resonances that result in nonthreshold electrons, there is a need for a high degree of rejection of such electrons, and the effectiveness with which this is accomplished depends upon the electron optical properties of the analyzer, lenses, and electrode surfaces. Separation of such resonant from nonresonant contributions to the partial cross sections for each ionic state is therefore very difficult, but when achieved can significantly increase our understanding of the dominant processes present within an atom or molecule. The introduction of pulsed field ionization‐zero kinetic energy (PFI-ZEKE) photoelectron spectroscopy in recent years [1] has largely overcome this problem and enabled a high-resolution spectroscopic probe of ionic states formed by excitation and subsequent field ionization.

We demonstrate a new spectroscopic method for studying electronic transitions in molecules with extremely broad range of angular momentum. We employ an optical centrifuge to create narrow rotational wave packets in the ground electronic state of $^{16}$O$_2$. Using the technique of resonance-enhanced multi-photon ionization, we record the spectrum of multiple ro-vibrational transitions between $X^3\Sigma_{g}^{-}$ and $C^3\Pi_{g}$ electronic manifolds of oxygen. Direct control of rotational excitation, extending to rotational quantum numbers as high as $N\gtrsim 120$, enables us to interpret the complex structure of rotational spectra of $C^3\Pi_{g}$ beyond thermally accessible levels., Comment: 5 pages, 6 figures

The energetic thresholds for ion-pair photodissociation of diatomic oxygen [O{sub 2} +h{nu}{r_arrow}O{sup +}+O{sup {minus}}({sup 2}P{sub 3/2}, {sup 2}P{sub 1/2})] were measured using a novel technique. The technique depends on the production and selective detection of highly vibrationally excited ion-pair states which last for at least 5 {mu}s following single photon excitation from the ground electronic state of O{sub 2} . These ion-pair states are analogous to Rydberg states, and they may be dissociated by an electric field in a similar manner to the field ionization of high principal quantum number Rydberg states, allowing for their selective detection. {copyright} {ital 1997} {ital The American Physical Society}

To examine the issues concerning Rydberg-state stabilization in pulsed-field ionization zero-kinetic-energy (PFI-ZEKE) photoelectron spectroscopy, spectra at the first and second ionization thresho...

A new experimental apparatus is described which permits the determination of binding energetics for metal−ligand complexes. This technique mates laser ablation for the generation of atomic metal ions with the environment of a high-pressure ion source which leads to rapid termolecular stabilization of metal ion-ligand complexes containing one or more ligands. The time resolved capability of the detection system allows equilibrium to be studied quantitatively for binding energies in the range of 5−30 kcal mol-1. Such measurements of the absolute binding energy of formaldehyde to Al+ combined with existing bimolecular Al+ exchange equilibrium data leads to an absolute Al+ affinity scale. For two ligand complexes an extensive ab initio search of the potential energy surfaces shows that the experimentally observed species involving CH3CN and (CH3)2O involve simple ligand complexation with an acute L-Al−L bond angle (L = ligand) rather than hydrogen bonded or inserted structures. In one instance a third ligand ...

The PFI-ZEKE spectra of the first four vibrational levels of O2+(a 4Uu) are reported. The rotationally resolved photoelectron spectra were assigned by diagonalizing the appropriate Hamiltonian in a Hund's case a basis set. An extension of a previous model for rotational line strengths (the BOS model) to the intermediate coupling case was applied to the three vibrational bands with ν+ = 1–3. As has been observed with several 2II states in previous ZEKE studies, the intensities of the spin-orbit sub-levels were not in accord with calculated line strengths. The a 4IIu vibrational levels ν+ = 0 and 1 were overlapped by the FranckCondon forbidden ν = 21 and 22 levels of the X2IIg state, allowing for a direct comparison between the intensities of the Franck-Condon allowed and forbidden bands in the ZEKE spectra. These data allowed us to determine an accurate ionization energy for the a 4IIu state: 129892 ± 2 cm−1 (16.1046 eV).

An experimental method is presented which combines state selection of reactant ions by threshold photoelectron-photoion coincidence following VUV photoionization with radiofrequency octopole ion guides. Absolute cross sections were measured for the charge transfer between state-selected Ar+(2Pj) ions and molecular oxygen as a function of collision energy in the 0.4–4 eV (CM) range. A critical analysis of the absolute values of both this study and studies reported in the literature is given, in which possible artifacts are identified mainly from coincidence time-of-flight spectra of O2+ product ions. Charge transfer mechanisms are discussed as a function of collision energy and fine structure state of Ar+ reactant ions.

The photoelectron angular distributions from autoionizing $\mathrm{ns}$ and $\mathrm{nf}$ Rydberg states of NO are presented. A striking dependence of the angular distribution on the rotational level in the excited Rydberg complex is observed: The sign of the asymmetry parameter $\ensuremath{\beta}$ alternates for consecutive rotational levels. A multichannel quantum defect calculation shows qualitative agreement with the experimental results and indicates the role of predissociation in the decay of these Rydberg states.

The rotationally resolved zero-kinetic-energy electron spectra for the ${\mathit{v}}^{+}$=1, 2, and 3 vibrational levels of the predissociating ${\mathit{A}}^{2}$${\mathrm{\ensuremath{\Sigma}}}^{+}$ state of ${\mathrm{HBr}}^{+}$ are reported. The results for the rapidly predissociating ${\mathit{v}}^{+}$=2 and 3 levels show that the high principal quantum number Rydberg states involved in the zero-kinetic-energy spectroscopy are unperturbed by ion core fragmentation. These data demonstrate the general applicability of this technique for rotationally resolved photoelectron spectroscopy.

We investigate experimentally the effect of quantum resonance in the rotational excitation of the simplest quantum rotor - a diatomic molecule. By using the techniques of high-resolution femtosecond pulse shaping and rotational state-resolved detection, we measure directly the amount of energy absorbed by molecules interacting with a periodic train of laser pulses, and study its dependence on the train period. We show that the energy transfer is significantly enhanced at quantum resonance, and use this effect for demonstrating selective rotational excitation of two nitrogen isotopologues, $ ^{14}N_2$ and $ ^{15}N_2$. Moreover, by tuning the period of the pulse train in the vicinity of a fractional quantum resonance, we achieve spin-selective rotational excitation of para- and ortho-isomers of $ ^{15}N_2$., Comment: 5 pages, 4 figures

The rotationally resolved photoelectron spectrum of high vibrational levels of O2+ in the Franck–Condon gap was investigated using pulsed field ionization, zero kinetic energy photoelectron spectroscopy. By using a coherent extreme ultraviolet light source for single-photon excitation, the ν+ = 6–24 vibrational levels of O2+X2Πg were studied. This is the first time levels higher than ν+ = 14 have been observed with rotational resolution. The highest level studied in the present work had a vibrational energy corresponding to 70% of the well depth. Along with the novelty of the spectroscopic technique, the present results reveal interesting and new ionization dynamics. All levels observed are Franck–Condon forbidden, and are not observed in a conventional photoelectron spectrum. There was no direct relation between the vibrational bands and the autoionizing states observed in the photoionization efficiency spectrum in the same energy region, and the rotational line intensities observed in the various bands in the current threshold photoelectron spectra were all similar. The mechanism of this process was different from the "resonant autoionization" model, which involves coupling between a Rydberg state with a low principal quantum number, a dissociative state, arid the Rydberg series converging to the vibrationally excited ion states. Instead, the excitation process was believed to be more direct, involving mainly a dissociative state (or states) and the Rydberg states with a vibrationally excited ion core. Further investigation of this mechanism is still necessary, but the formation of these highly vibrationally excited ions opens a new horizon in state-selective reaction dynamics. With the coherent XUV light source and the PFI-ZEKE technique, a wide range of vibrational energies (up to 4 eV) can be deposited into the O2+ reactant with rovibrational selectivity.

We report results on the pulsed field ionization threshold photelectron spectroscopy of the A 2 Σ + state of N 2 O + , using coherent extreme ultraviolet for single photon ionization. For variable delays up to 1 μs between excitation and ionization no significant change in the threshold signal was observed, in spite of the 250 ns radiative lifetime of the excited ion core. The rotational profiles of the (000) and (100) bands were simulated by the standard model for rotational linestrengths in photoionization, and these results reveal information on the dynamics of the Rydberg states involved in pulsed field ionization.

The high-resolution threshold photoelectron spectrum of the HF molecule has been measured using pulsed field ionization after single-photon excitation by coherent vacuum ultra-violet radiation. The adiabatic ionization potential for HF was found to be 129422.4±1 cm −1 . In contrast to HCl, only small changes in total angular momentum of the ion core are observed. The experimental results show a preference for the F 1 spin—orbit component for both vibrational states of the ion, whereas the vibrational branching ratio was found to be similar to the relative intensities obtained from the Franck-Condon principle.

Rabi oscillations of quantum population are known to occur in two-level systems driven by spectrally narrow laser fields. In this work we study Rabi oscillations induced by shaped broadband femtosecond laser pulses. Due to the broad spectral width of the driving field, the oscillations are initiated between a ground state and a coherent superposition of excited states, or a "wavepacket", rather than a single excited state. Our experiments reveal an intricate dependence of the wavepacket phase on the intensity of laser field. We confirm numerically that the effect is associated with the strong-field nature of the interaction, and provide a qualitative picture by invoking a simple theoretical model., 6 pages, 6 figures

Understanding the effect of strong laser pulses on the evolution of an atomic or molecular wave function is important in the context of coherent control in the strong-field regime, when power broadening and dynamic Stark shifts become comparable with or bigger than the bandwidth of the control field. We experimentally demonstrate the method of complete characterization of a complex-valued amplitude of a quantum state driven by a strong two-photon field. The method is based on coherent scattering of a weak probe pulse from the strong-field-induced atomic coherence, followed by the detection of the time- and frequency-resolved parametric four-wave-mixing signal. We show that the proposed technique corresponds to a cross-correlation frequency-resolved optical gating (XFROG) of the highly perturbed evolution of an atomic quantum state. Utilizing the XFROG retrieval algorithm, we determine both the amplitude and phase of an atomic wave function at any time moment throughout the interaction with the driving field. The direct retrieval of the time-dependent phase of the wave function, rather than the population dynamics only, enables us to observe the strong-field effects with arbitrary time and frequency resolution.

(35 Word Limit) Frequency resolved gating of laser-induced atomic coherence enables complete reconstruction of the quantum evolution of atomic and molecular systems driven by laser fields. The method is applied to characterization and control of laser-matter interactions.

We report the observation of Franck—Condon forbidden highly vibrationally excited states of the oxygen molecular ion through threshold pulsed field ionization spectroscopy. The spectra were recorded using a broadly tunable coherent XUV light source for single-photon excitation of supersonically cooled oxygen. The rotational band contours are well described by the standard model for rotational linestrengths in photoelectron spectra, and are similar to those observed previously for Franck—Condon allowed bands.

The vibrational, rotational, and translational energy distributions for the CO photofragments from the 193, 248, and 308 nm photolysis of Ni(CO)4 in a supersonic molecular beam have been determined by vacuum ultraviolet laser‐induced fluorescence. The measured product energy distributions appeared to be statistical, with equilibrium between the degrees of freedom investigated. The distributions were significantly colder than those calculated with a microcanonical statistical model using published bond energies. To model the measured distributions, it was necessary to postulate that the unsaturated nickel carbonyl products are formed in a stable electronically excited state. By using an excited state energy consistent with published fluorescence experiments, excellent agreement was obtained between the measured distributions and those calculated using a microcanonical statistical model. These results indicate that for 193 nm photolysis, essentially all of the Ni(CO)n products are electronically excited, wi...

This paper presents the studies of one and two component particles using a CO(2) laser for vaporization and VUV ionization in an ion trap mass spectrometer. The degree of fragmentation for a one component system was demonstrated to be a function of CO(2) laser energy. In a two component system, the degree of fragmentation was shown to be a function of the particle composition. This observation indicates that the analysis of mixed particles may be far more complicated than anticipated for a two step process with soft vaporization. In addition to showing that fragmentation is a function of CO(2) laser energy and particle composition, we also show that a key parameter that determines the extent of fragmentation is the energy absorbed by the particle during desorption. The ionization delay profile in a one component system is also shown to be strongly dependent on the vaporization energy. In a two component system, the delay profile is shown to strongly depend on the composition of the particle. The combined data suggest that the key parameter that governs the delay profile is the energy absorbed by the particle during desorption. This finding has implications for potential field measurements. Finally, for a two component system where the absorption crosssections are different, the change in the degree of fragmentation with particle composition resulted in a non-linear dependence of ion signal on composition. This makes any attempt at quantification difficult.

An investigation of oleic acid and 2,4-dihydroxybenzoic (DHB) acid aerosols was carried out using an aerosol mass spectrometer with pulsed lasers for vaporization and ionization and an ion trap for mass analysis. The extent of ion fragmentation was studied as a function of both vaporization energy and ionization wavelength. Low CO2 laser energies in the vaporization stage and near-threshold single photon ionization resulted in the least fragmented mass spectra. For DHB, only the molecular ion was observed, but for oleic acid fragmentation could not be eliminated. Tandem MS of the main fragment peak from oleic acid was carried out and provided a tool for compound identification. Photoionization efficiency curves were also collected for both DHB and oleic acid and the appearance energies of both parent and fragment ions were measured. Evidence for fragmentation occurring post-ionization is given by the similar appearance energies for both the parent and fragment ions. The results from this study were compared with those from similar experiments undertaken with time-of-flight (TOF) mass analyzers. The degree of fragmentation in the ion trap was considerably higher than that seen with TOF systems, particularly for oleic acid. This was attributed to the long storage interval in the ion trap which allows time for metastable ions to decay. Differences in the degree of fragmentation between the ion trap and TOF studies also provided further evidence for fragmentation occurring post-ionization. For 2,4-dihydroxybenzoic acid, the long delay prior to mass analysis also allowed time for reactions with background gases, in this case water, to occur.

High-peak-power ultrafast lasers are widely used in nonlinear spectroscopy, but often limit its spectral resolution because of the broad frequency bandwidth of ultrashort laser pulses. Improving the resolution by achieving spectrally narrow excitation of, or emission from, the resonant medium by means of multiphoton interferences has been the focus of many recent developments in ultrafast spectroscopy. We demonstrate an alternative approach in which high resolution is exercised by detecting narrow spectral correlations between broadband excitation and emission optical fields. All-optical correlation analysis, easily incorporated into a traditional spectroscopic setup, enables direct, robust, and simultaneous detection of multiple narrow resonances in a single measurement.

Initial results from direct clustering measurements of enthalpies and entropies of binding of acetone to Al+ using a high pressure mass spectrometer led to the hypothesis that ligandligand bonding was involved in Al(acetone)+2. In this study, however, collision-induced dissociation, ligand-displacement reactions, and infrared multiphoton photodissociation of the mixed species Al(acetone)(acetone-d6)+ in a Fourier transform mass spectrometer show that the two acetone ligands in Al(acetone)+2 must be equivalent. Ab initio calculations support the conclusion that there are no ligandligand bonds, but that polarisation of the Al+ leads to both acetones binding on the same side. The resulting ligandligand interaction explains the difference in the observed entropy for the Al(acetone)+ system and those of other simple association reactions. Finally, direct clustering results from a newly constructed pulsed laser ionisation high pressure mass spectrometer are in excellent agreement with the Fourier transform mass spectrometry and ab initio findings.

We report the observation of above-threshold dissociation of a multielectron molecular system, $\mathrm{Na}_{2}{}^{+}$. The process is due to continuum-continuum transitions in a field-dressed molecular continuum. Three well-resolved fragmentation channels with different kinetic energy release patterns have been detected, of which two are associated with ordinary photodissociation via ``light-induced potentials'' into $\mathrm{Na}(3s)+{\mathrm{Na}}^{+}$ and $\mathrm{Na}(3p)+{\mathrm{Na}}^{+}$, and one with continuum-continuum transitions and above-threshold dissociation into $\mathrm{Na}(3p)+{\mathrm{Na}}^{+}$. We show that when we dress this molecule with a field whose intensity is $\ensuremath{\sim}3\ifmmode\times\else\texttimes\fi{}{10}^{12}\phantom{\rule{0.3em}{0ex}}\mathrm{W}∕{\mathrm{cm}}^{2}$, above-threshold dissociation prevails over other processes, such as field ionization followed by Coulomb explosion.

The vibrational, rotational, and translational energy distributions for the CO photofragments from the 193, 248, and 308 nm photolysis of Ni(CO)4 in a supersonic molecular beam have been determined by vacuum ultraviolet laser‐induced fluorescence. The measured product energy distributions appeared to be statistical, with equilibrium between the degrees of freedom investigated. The distributions were significantly colder than those calculated with a microcanonical statistical model using published bond energies. To model the measured distributions, it was necessary to postulate that the unsaturated nickel carbonyl products are formed in a stable electronically excited state. By using an excited state energy consistent with published fluorescence experiments, excellent agreement was obtained between the measured distributions and those calculated using a microcanonical statistical model. These results indicate that for 193 nm photolysis, essentially all of the Ni(CO)n products are electronically excited, wi...

We propose and demonstrate the method of population transfer by piecewise adiabatic passage between two quantum states. Coherent excitation of a two-level system with a train of ultrashort laser pulses is shown to reproduce the effect of an adiabatic passage, conventionally achieved with a single frequency-chirped pulse. By properly adjusting the amplitudes and phases of the pulses in the excitation pulse train, we achieve complete and robust population transfer to the target state. The effect is demonstrated experimentally by observing piecewise excitation of Rubidium atoms from 5s_1/2 to 5p_1/2 electronic state. We show that similarly to the conventional adiabatic passage, the piecewise process is insensitive to the total excitation energy as long as the adiabaticity conditions are satisfied. The piecewise nature of the process suggests that robust and selective population transfer could be implemented in a variety of complex quantum systems beyond the two-level approximation., Submitted to Phys. Rev. Lett

Hydrogen is efficiently released during water dissociation on zirconium (Zr), while even very rapid temperature programmed heating of a hydrogen covered Zr surface predominantly leads to dissolution (approximately 99% dissolution). To help resolve these apparently contradictory observations, we have studied the dynamics of water (D2O) dissociation on a crystalline Zr surface by probing the rotational and vibrational energy distributions of the D2 produced using resonant enhanced multiphoton ionization spectroscopy. The internal-state energy distribution of the D2 product was found to be rotationally cold and vibrationally hot with respect to the temperature of the surface. The rotational distribution shows slight deviations from Boltzmann's law, with a mean rotational temperature of 426 K while the surface is at 800 K. The population of the nu"=1 vibration is at least four times higher than a 800 K temperature would allow, this corresponding to a vibrational temperature of 1100 K. Information on the translational energy of the D2 product have also been obtained by time-of-flight spectroscopy and it is found to be nearly thermally equilibrated with the surface temperature. Similar results were obtained from studies of D2 scattered from a clean Zr surface, and of D2 released by a slow thermal desorption process which involves dissolved hydrogen as the source. The reconciliation of the present results with those for thermal desorption of preadsorbed hydrogen implies a role for both surface and subsurface adsorption sites on the Zr surface and clearly demonstrates that at high temperatures, the release of D2 arises from the recombinative desorption of adsorbed hydrogen formed by the complete dissociation of D2O.

The spectroscopic technique of threshold ion-pair production spectroscopy (TIPPS) has been applied to the triatomic molecule HCN. We have recorded the total ion-pair yield and TIPP spectra for the HCN-->H(+) + CN(-) process using coherent vacuum ultraviolet excitation. From the simulation of our high-resolution TIPP spectrum we have precisely measured the HCN ion-pair threshold E(IP) (0) to be 122 244 +/- 4 cm(-1). This value could be used to determine the bond dissociation energy D(0)(H-CN) to unprecedented accuracy. Our fitting result also showed that rotationally excited instead of cold CN(-) fragment is favored as the ion-pair dissociation product in the threshold region.

The introduction of the pulsed field ionization zero kinetic energy photoelectron spectroscopy technique (referred to as PFI-ZEKE spectroscopy) has resulted in a revolution in photoelectron spectroscopy, because of the tremendous improvement in resolution. This method of threshold photoelectron spectroscopy is based on field ionization of metastable high principal quantum number Rydberg states using a pulsed electric field, delayed from the laser excitation. The detailed mechanism for stabilization of the high principal quantum number Rydberg states has been the subject of a great deal of recent discussion in the literature, and is still somewhat controversial. It is well known that Rydberg state lifetimes scale as n-3, for fluorescence, autoionization, or predissociation, under ideal conditions. This means that for a Rydberg series that can decay by autoionization, if the lifetime of a 5p Rydberg state is 10-12 s, the lifetime of a 150p state will be 10-7 s, which is an order of magnitude shorter than typical delay times used in PFI-ZEKE. The 150p state will be field ionized by an electric field of 0.7 to 1.5 V/cm, which is typical of the pulsed fields used for Stark ionization. This question about Rydberg state lifetimes becomes quite important if one wishes to carry out PFI-ZEKE spectroscopy of ion states well above the lowest ionization threshold, as many decay channels will be available to the Rydberg states converging to the high energy states, resulting in shorter lifetimes for these high energy Rydberg states. Our work in this area has focused largely on PFI-ZEKE spectroscopy at excited state thresholds in molecular ions, where problems of autoionization will be most severe. To reach these high energy thresholds, we have usually used single photon excitation with coherent vacuum ultraviolet light. This excitation method has many advantages.© (1995) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

\We report results on pulsed field ionization of NO and O2. The experiments were carried out using tunable coherent vacuum ultraviolet radiation for single photon excitation of the NO or O2 molecules. In the case of NO, light between 15.66 eV and 15.90 eV was used to probe the v equals 0, 1, and 2 levels of the NO+(a3(Sigma) +) state. The resulting PFI-ZEKE spectra were interpreted using a model developed to explain rotational line intensities in photoelectron spectra. The rotational line intensities were found to vary systematically with v+, in disagreement with the model, and the intensities in the v+ equals 1 band were strongly affected by the presence of a complex resonance at that energy. In addition, the relative intensities of the v+ equals 0, 1, and 2 bands were very different from the Franck-Condon factors, with the intensity of the v+ equals 1 band strongly enhanced by the complex resonance, and the v+ equals 2 band having a much smaller intensity than expected. In oxygen, we were able to record rotationally resolved PFI-ZEKE spectra of the v+ equals 6 to 21 vibrational levels of the O2+(X2$PRDg) electronic state. As these levels have zero Franck-Condon factors with O2(X3(Sigma) g+)(v equals 0), their observation shows the presence of a type of `resonant autoionization,' where neutral continuum states provide a coupling between Franck-Condon allowed Rydberg states and the Franck-Condon forbidden levels observed.© (1993) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Although there has been a tremendous growth in research using pulsed field ionization to record ultra-high resolution threshold photoelectron spectra of molecules, most work has concentrated on the lowest ionization thresholds in small molecules.

We study vibrational dynamics of molecules interacting with spectrally shaped broadband laser pulses. After performing a single measurement based on cross-correlation frequency resolved optical gating of molecular vibration, complete evolution of the complex-valued quantum coherence between the vibrational states is reconstructed with variable time and frequency resolution. The ability to change the resolution in the analysis of the transient molecular dynamics without repeating the experiment or changing experimental parameters is useful in designing and understanding various schemes of controlling quantum states of molecules.