Photofragment Ions Research Articles

Demonstration of Capture, Cooling, Tagging, and Spectroscopic Characterization of UV Photoproduct Ions in a Cryogenic Ion Trap: Application to 266 nm Photofragment Ions from Rhodamine 6G.

We demonstrate a method to determine the structures of the primary photodissociation products from a cryogenically cooled parent ion. In this approach, a target ion is cooled by a pulse of buffer gas and tagged in a 20 K Paul trap. The cold ion is then photodissociated by pulsed (∼5 ns) UV laser excitation, and the ionic products are trapped, cooled, and tagged by introduction of a second buffer gas pulse in the same trap. The tagged fragments are then ejected into a triple focusing, UV/vis/IR time-of-flight photofragmentation mass spectrometer which yields vibrational and electronic spectra of the mass-selected photofragments. These methods are demonstrated by application to the 266 nm photodissociation of the Rhodamine 6G cation to yield the R575 fragment ion based on loss of ethene as well as to a weaker secondary fragment arising from loss of m/z 43.

Ionization energies and ionization-induced structural changes in 2-phenylethylamine and its monohydrate.

We report the resonance-enhanced two-photon ionization combined with various detection approaches and quantum chemical calculations of biologically relevant neurotransmitter prototypes, the most stable conformer of 2-phenylethylamine (PEA), and its monohydrate, PEA-H2O, to reveal the possible interactions between the phenyl ring and amino group in the neutral and ionic species. Extracting the ionization energies (IEs) and appearance energy was achieved by measuring the photoionization and photodissociation efficiency curves of the PEA parent and photofragment ions, together with velocity and kinetic energy-broadened spatial map images of photoelectrons. We obtained coinciding upper bounds for the IEs for PEA and PEA-H2O of 8.63 ± 0.03 and 8.62 ± 0.04eV, within the range predicted by quantum calculations. The computed electrostatic potential maps show charge separation, corresponding to a negative charge on phenyl and a positive charge on the ethylamino side chain in the neutral PEA and its monohydrate; in the cations, the charge distributions naturally become positive. The significant changes in geometries upon ionization include switching of the amino group orientation from pyramidal to nearly planar in the monomer but not in the monohydrate, lengthening of the N-H⋯π hydrogen bond (HB) in both species, Cα-Cβ bond in the side chain of the PEA+ monomer, and the intermolecular O-H⋯N HB in PEA-H2O cations, leading to distinct exit channels.

Ultraviolet photodissociation of Mg+-NO complex: Ion imaging of a reaction branching in the excited states.

Ultraviolet photodissociation processes of gas phase Mg+-NO complex were studied by photofragment ion imaging experiments and theoretical calculations for excited electronic states. At 355nm excitation, both Mg+ and NO+ photofragment ions were observed with positive anisotropy parameters, and theoretical calculations revealed that the two dissociation channels originate from an electronic transition from a bonding orbital consisting of Mg+ 3s and NO π* orbitals to an antibonding counterpart. For the NO+ channel, the photofragment image exhibited a high anisotropy (β = 1.53 ± 0.07), and a relatively large fraction (∼40%) of the available energy was partitioned into translational energy. These observations are rationalized by proposing a rapid dissociation process on a repulsive potential energy surface correlated to the Mg(1S) + NO+(1Σ) dissociation limit. In contrast, for the Mg+ channel, the angular distribution was more isotropic (β = 0.48 ± 0.03) and only ∼25% of the available energy was released into translational energy. The differences in the recoil distribution for these competing channels imply a reaction branching on the excited state surface. On the theoretical potential surface of the excited state, we found a deep well facilitating an isomerization from bent geometry in the Franck-Condon region to linear and/or T-shaped isomer. As a result, the Mg+ fragment was formed via the structural change followed by further relaxation to lower electronic states correlated to the Mg+(2S) + NO(2Π) exit channel.

Femtosecond XUV–IR induced photodynamics in the methyl iodide cation

The time-resolved photodynamics of the methyl iodide cation (CH3I+) are investigated by means of femtosecond XUV–IR pump–probe spectroscopy. A time-delay-compensated XUV monochromator is employed to isolate a specific harmonic, the 9th harmonic of the fundamental 800 nm (13.95 eV, 88.89 nm), which is used as a pump pulse to prepare the cation in several electronic states. A time-delayed IR probe pulse is used to probe the dissociative dynamics on the first excited state potential energy surface. Photoelectrons and photofragment ions— and I+—are detected by velocity map imaging. The experimental results are complemented with high level ab initio calculations for the potential energy curves of the electronic states of CH3I+ as well as with full dimension on-the-fly trajectory calculations on the first electronically excited state , considering the presence of the IR pulse. The and I+ pump–probe transients reflect the role of the IR pulse in controlling the photodynamics of CH3I+ in the state, mainly through the coupling to the ground state and to the excited state manifold. Oscillatory features are observed and attributed to a vibrational wave packet prepared in the state. The IR probe pulse induces a coupling between electronic states leading to a slow depletion of fragments after the cation is transferred to the ground states and an enhancement of I+ fragments by absorption of IR photons yielding dissociative photoionization.

Photofragment ion imaging in vibrational predissociation of the H2O+Ar complex ion.

Vibrational predissociation processes of the H2O+Ar complex ion following mid-infrared excitations of the OH stretching modes and bending overtone of the H2O+ unit were studied by photofragment ion imaging. The anisotropy parameters, β, of the angular distributions of the photofragment ions were clearly dependent on the type (branch) of rotational excitation, β > 0 for the P-branch excitations, while β < 0 for the Q-branch excitations, which were consistent with the previous theoretical predictions for the rotationally resolved optical transition of a prolate symmetric top. The translational energy distributions had a similar form, irrespective of the excitation modes. This result suggests that the prepared excited states underwent a common relaxation pathway via the bending or bending overtone state of the H2O+ unit. In addition, the available energy was preferentially distributed into the rotational energy of the H2O+ fragment ions rather than the translational energy. The mechanism of the rotational excitations of the H2O+ fragment ions was discussed based on the steric configuration of the H2O+ and Ar units at the moment of dissociation.

Electronic Spectrum and Photodissociation Chemistry of the 1-Butyn-3-yl Cation, H3CCHCCH.

The B̃1A' ← X̃1A' electronic spectra of the 1-butyn-3-yl cation (H3CCHCCH+) and the H3CCHCCH+-Ne and H3CCHCCH+-Ar complexes are measured using resonance enhanced photodissociation over the 245-285 nm range, with origin transitions occurring at 35936, 35930, and 35928 cm-1, respectively. Vibronic bands are assigned based on quantum chemical calculations and comparison of the spectra with those of the related linear methyl propargyl (H3C4H2+) and propargyl (H2C3H+) cations. The photofragment ions are C2H3+ (major) and C4H3+ (minor), with the preference for C2H3+ consistent with master equation simulations for a mechanism that involves rapid electronic deactivation and dissociation on the ground state potential energy surface.

A mini-photofragment translational spectrometer with ion velocity map imaging using low voltage acceleration.

A mini time-sliced ion velocity map imaging photofragment translational spectrometer using low voltage acceleration has been constructed. The innovation of this apparatus adopts a relative low voltage (30-150 V) to substitute the traditional high voltage (650-4000 V) to accelerate and focus the fragment ions. The overall length of the flight path is merely 12 cm. There are many advantages for this instrument, such as compact structure, less interference, and easy to operate and control. Low voltage acceleration gives a longer turn-around time to the photofragment ions forming a thicker Newton sphere, which provides sufficient time for slicing. Ion trajectory simulation has been performed for determining the structure dimensions and the operating voltages. The photodissociation and multiphoton ionization of O2 at 224.999 nm is used to calibrate the ion images and examine the overall performance of the new spectrometer. The velocity resolution (Δν/ν) of this spectrometer from O2 photodissociation is about 0.8%, which is better than most previous results using high acceleration voltage. For the case of CF3I dissociation at 277.38 nm, many CF3 vibrational states have been resolved, and the anisotropy parameter has been measured. The application of low voltage acceleration has shown its advantages on the ion velocity map imaging (VMI) apparatus. The miniaturization of the VMI instruments can be realized on the premise of high resolution.

Development of a linear-type double reflectron for focused imaging of photofragment ions from mass-selected complex ions.

An ion imaging apparatus with a double linear reflectron mass spectrometer has been developed, in order to measure velocity and angular distributions of mass-analyzed fragment ions produced by photodissociation of mass-selected gas phase complex ions. The 1st and the 2nd linear reflectrons were placed facing each other and controlled by high-voltage pulses in order to perform the mass-separation of precursor ions in the 1st reflectron and to observe the focused image of the photofragment ions in the 2nd reflectron. For this purpose, metal meshes were attached on all electrodes in the 1st reflectron, whereas the mesh was attached only on the last electrode in the 2nd reflectron. The performance of this apparatus was evaluated using imaging measurement of Ca+ photofragment ions from photodissociation reaction of Ca+Ar complex ions at 355 nm photoexcitation. The focused ion images were obtained experimentally with the double linear reflectron at the voltages of the reflection electrodes close to the predictions by ion trajectory simulations. The velocity and angular distributions of the produced Ca+ ([Ar] 4p1, 2P3/2) ion were analyzed from the observed images. The binding energy D0 of Ca+Ar in the ground state deduced in the present measurement was consistent with those determined theoretically and by spectroscopic measurements. The anisotropy parameter β of the transition was evaluated for the first time by this instrument.

PHOTO-STABILITY OF SUPER-HYDROGENATED PAHs DETERMINED BY ACTION SPECTROSCOPY EXPERIMENTS

ABSTRACT We have investigated the photo-stability of pristine and super-hydrogenated pyrene cations ( ) by means of gas-phase action spectroscopy. Optical absorption spectra and photo-induced dissociation mass spectra are presented. By measuring the yield of mass-selected photo-fragment ions as a function of laser pulse intensity, the number of photons (and hence the energy) needed for fragmentation of the carbon backbone was determined. Backbone fragmentation of pristine pyrene ions ( ) requires absorption of three photons of energy just below 3 eV, whereas super-hydrogenated hexahydropyrene (C16H ) must absorb two such photons and fully hydrogenated hexadecahydropyrene (C16H ) only a single photon. These results are consistent with previously reported dissociation energies for these ions. Our experiments clearly demonstrate that the increased heat capacity from the additional hydrogen atoms does not compensate for the weakening of the carbon backbone when pyrene is hydrogenated. In photodissociation regions, super-hydrogenated Polycyclic Aromatic Hydrocarbons (PAHs) have been proposed to serve as catalysts for H2 formation. Our results indicate that carbon backbone fragmentation may be a serious competitor to H2 formation at least for small hydrogenated PAHs like pyrene.

Nile blue shows its true colors in gas-phase absorption and luminescence ion spectroscopy.

Nile blue is used extensively in biology as a histological stain and fluorescent probe. Its absorption and emission spectra are strongly solvent dependent, with variations larger than 100 nm. The molecule is charged due to an iminium group, and it is therefore an obvious target for gas-phase ion spectroscopy. Here we report the absorption and emission spectra of the mass-selected bare ions isolated in vacuo, and based on our results we revisit the interpretation of solution-phase spectra. An accelerator mass spectrometer was used for absorption spectroscopy where the absorption is represented by the yield of photofragment ions versus excitation wavelength (action spectroscopy). The luminescence experiments were done with a newly built ion trap setup equipped with an electrospray ion source, and some details on the mass selection technique will be given which have not been described before. In vacuo, the absorption and emission maxima are at 580 ± 10 nm and 628 ± 1 nm. These values are somewhat blue-shifted relative to those obtained in most solvents; however, they are much further to the red than those in some of the most non-polar solvents. Furthermore, the Stokes shift in the gas phase (1300 cm(-1)) is much smaller than that in these non-polar solvents but similar to that in polar ones. An explanation based on charge localization by solvent dipoles, or by counterions in some non-polar solvents, can fully account for these findings. Hence in the case of ions, it is nontrivial to establish intrinsic electronic transition energies from solvatochromic shifts alone.

Three-dimensional imaging of carbonyl sulfide and ethyl iodide photodissociation using the pixel imaging mass spectrometry camera.

The Pixel Imaging Mass Spectrometry (PImMS) camera is used in proof-of-principle three-dimensional imaging experiments on the photodissociation of carbonyl sulfide and ethyl iodide at wavelengths around 230 nm and 245 nm, respectively. Coupling the PImMS camera with DC-sliced velocity-map imaging allows the complete three-dimensional Newton sphere of photofragment ions to be recorded on each laser pump-probe cycle with a timing precision of 12.5 ns, yielding velocity resolutions along the time-of-flight axis of around 6%-9% in the applications presented.

Excited State Dynamics of Protonated Phenylalanine and Tyrosine: Photo-Induced Reactions Following Electronic Excitation.

The electronic spectroscopy and the electronic excited state properties of cold protonated phenylalanine and protonated tyrosine have been revisited on a large spectral domain and interpreted by comparison with ab initio calculations. The protonated species are stored in a cryogenically cooled Paul trap, maintained at ∼10 K, and the parent and all the photofragment ions are mass-analyzed in a time-of-flight mass spectrometer, which allows detecting the ionic species with an improved mass resolution compared to what is routinely achieved with a quadrupole mass spectrometer. These new results emphasize the competition around the band origin between two proton transfer reactions from the ammonium group toward either the aromatic chromophore or the carboxylic acid group. These reactions are initiated by the coupling of the locally excited ππ* state with higher charge transfer states, the positions and coupling of which depend on the conformation of the protonated molecules. Each of these reaction processes gives rise to specific fragmentation channels that supports the conformer selectivity observed in the photofragmentation spectra of protonated tyrosine and phenylalanine.

Ejection process of photofragment ions from a quadrupole ion trap

We investigate the ejection process of photofragment ions from a quadrupole ion trap (QIT) that occurs immediately after irradiation of dibenzo-18-crown-6 complexes with alkali metal cations (M+-DB18C6, M=K, Rb, and Cs) in the QIT by an ultraviolet (UV) laser pulse. The first detection of those fragment ions escaping from the QIT using a reflectron time-of-flight (reTOF) mass spectrometer attached next to the QIT provides a basis for our recent development of a new UV–UV hole burning (HB) spectroscopic technique applicable to ions in a QIT. To understand the ejection process, we measure the dependences of the ejection time of the M+ ions on the rf amplitude, time delay between the laser and the extraction direct current (DC) pulse, and buffer gas pressure inside the QIT. We suggest that the photofragment ions undergo unstable oscillatory motion upon their generation in the QIT due to their larger values of the Mathieu parameter qz than that of the stability boundary. The fragment ions increase their oscillation amplitudes by taking up energy from the rf field and are eventually ejected out of the QIT. The lower limits of the qz values necessary for the escape of K+, Rb+, and Cs+ ions from the QIT are determined to be ∼1.7.

Hybrid quadrupole mass filter/quadrupole ion trap/time-of-flight-mass spectrometer for infrared multiple photon dissociation spectroscopy of mass-selected ions

We present a laboratory-constructed mass spectrometer optimized for recording infrared multiple photon dissociation (IRMPD) spectra of mass-selected ions using a benchtop tunable infrared optical parametric oscillator∕amplifier (OPO∕A). The instrument is equipped with two ionization sources, an electrospray ionization source, as well as an electron ionization source for troubleshooting. This hybrid mass spectrometer is composed of a quadrupole mass filter for mass selection, a reduced pressure (∼10(-5) Torr) quadrupole ion trap (QIT) for OPO irradiation, and a reflectron time-of-flight drift tube for detecting the remaining precursor and photofragment ions. A helium gas pulse is introduced into the QIT to temporarily increase the pressure and hence enhance the trapping efficiency of axially injected ions. After a brief pump-down delay, the compact ion cloud is subjected to the focused output from the continuous wave OPO. In a recent study, we implemented this setup in the study of protonated tryptophan, TrpH(+), as well as collision-induced dissociation products of this protonated amino acid [W. K. Mino, Jr., K. Gulyuz, D. Wang, C. N. Stedwell, and N. C. Polfer, J. Phys. Chem. Lett. 2, 299 (2011)]. Here, we give a more detailed account on the figures of merit of such IRMPD experiments. The appreciable photodissociation yields in these measurements demonstrate that IRMPD spectroscopy of covalently bound ions can be routinely carried out using benchtop OPO setups.

Infrared spectra and density functional theory calculations for Mn +–(CH 4) n ( n = 1–6) clusters

Infrared spectra are measured for the mass-selected Mn(CH 4) n + ( n = 1–6) clusters in the C–H stretch region (2700–3100 cm −1) by monitoring photofragment ions. Accompanying density functional theory calculations are performed for possible structural isomers, including the Mn +–(CH 4) n ion-molecule complexes in which the ligands are attached to the Mn + ion in an η 3 configuration, and the inserted H–Mn–CH 3 +–(CH 4) n−1 complexes. The experimental infrared spectra for all species are dominated by a single band, appearing at 2836 cm −1 for n = 1 and progressively shifting to 2880 cm −1 for n = 6. The observed infrared spectra match predicted spectra for Mn +–(CH 4) n ion-molecule complexes containing Mn + in its 7S (4s 13d 5) electronic ground state, with the dominant band corresponding to the totally symmetric C–H stretching vibrations of the attached CH 4 ligands. No evidence was found for clusters containing the Mn + cation in its 5S (4s 13d 5) electronic state. In larger clusters, the CH 4 ligands are generally bound to the Mn + ion such that adjacent Mn +⋯C bonds are approximately at right angles to one another taking advantage of partial 4s/4p hybridization to minimise the CH 4 molecules’ repulsive interaction with the half-filled 4s orbital of Mn +.

Internal Energies of Ion-Sputtered Neutral Tryptophan and Thymine Molecules Determined by Vacuum Ultraviolet Photoionization

Vacuum ultraviolet photoionization coupled to secondary neutral mass spectrometry (VUV-SNMS) of deposited tryptophan and thymine films are performed at the Chemical Dynamics Beamline. The resulting mass spectra show that while the intensity of the VUV-SNMS signal is lower than the corresponding secondary ion mass spectroscopy (SIMS) signal, the mass spectra are significantly simplified in VUV-SNMS. A detailed examination of tryptophan and thymine neutral molecules sputtered by 25 keV Bi(3)(+) indicates that the ion-sputtered parent molecules have approximately 2.5 eV of internal energy. While this internal energy shifts the appearance energy of the photofragment ions for both tryptophan and thymine, it does not change the characteristic photoionizaton efficiency (PIE) curves of thymine versus photon energy. Further analysis of the mass spectral signals indicate that approximately 80 neutral thymine molecules and 400 tryptophan molecules are sputtered per incident Bi(3)(+) ion. The simplified mass spectra and significant characteristic ion contributions to the VUV-SNMS spectra indicate the potential power of the technique for organic molecule surface analysis.

Absolute cross-sections for dissociative photoionization of some small esters

The first measurements of absolute cross-sections for near-threshold molecular and dissociative photoionization are presented for 11 small esters (methyl formate, ethyl formate, vinyl acetate, methyl propanoate, ethyl propanoate, methyl butanoate, methyl isobutanoate, methyl propenoate, ethyl propenoate, methyl crotonate, and methyl methacrylate). Photoionization mass spectrometry (PIMS) is employed with an energy resolution of 40 meV (fwhm) using a monochromated VUV synchrotron light source. An extensive literature exploring isomerization/dissociation mechanisms is available for six of these molecules. There are, however, no previous observations of dissociative ionization for four simple monounsaturated esters (methyl propenoate, ethyl propenoate, methyl crotonate, and methyl methacrylate), and dissociative ionization of ethyl propanoate has received scant attention. Appearance energies for dissociative photofragment ions of these five molecules are presented.

Development of a Linear Ion Trap/Orthogonal-Time-of-Flight Mass Spectrometer for Time-Dependent Observation of Product Ions by Ultraviolet Photodissociation of Peptide Ions

A hybrid linear ion trap/orthogonal time-of-flight (TOF) mass spectrometer has been developed to observe time-dependent vacuum ultraviolet photodissociation product ions. In this apparatus, a reflectron TOF mass analyzer is orthogonally interfaced to an LTQ using rf-only octopole and dc quadrupole ion guides. Precursor ions are generated by electrospray ionization and isolated in the ion trap. Subsequently they are directed to the TOF source where photodissociation occurs and product ions are extracted for mass analysis. To detect photodissociation product ions having axially divergent trajectories, a large rectangular detector is utilized. With variation of the time between photodissociation and orthogonal extraction in the TOF source, product ions formed over a range of times after photoexcitation can be sampled. Time-dependent observation of product ions following 157 nm photodissociation of a singly charged tryptic peptide ion (NWDAGFGR) showed that prompt photofragment ions (x- and v-type ions) dominate the tandem mass spectrum up to 1 micros after the laser shot, but the intensities of low energy thermal fragment ions (y-type ions) become comparable several microseconds later. Different proton mobilization time scales were observed for arginine- and lysine-terminated tryptic peptides.

Photodissociation of vibrationally excited OH/OD radicals

This paper describes a joint experimental and theoretical study of the photodissociation of vibrationally excited hydroxyl radicals. OH and OD radicals produced in a pulsed electric discharge supersonic beam are state-selected and focused by a hexapole and then photo-dissociated by a single laser tuned to various H/D or O atom (2 + 1) resonance enhanced multiphoton ionization (REMPI) wavelengths between 243 nm and 200 nm. The angle velocity distributions of the resulting O+ and D+ photofragment ions were recorded using velocity map imaging. Photodissociation to the O(3PJ) + H(2S) limit is shown to take place by one-photon excitation to the repulsive 1 2Σ− state. The experimental data shows that vibrationally excited OH/OD which are formed in the discharge are dissociated, and a vibrational temperature of ≈2000 K was estimated for the beam source. An analysis in the high-energy recoil sudden limit is used to predict the O(3PJ) fine structure branching ratios and alignment information in the molecular and laboratory velocity frame of the imaging experiment. The measured and predicted fine structure branching ratios and alignment parameters agree well at all dissociation wavelengths, supporting the model for photodissociation in the sudden limit regime. Several aspects of the experiment such as OH pre-alignment and orientation, ion-recoil, and Doppler-free imaging are discussed.

Photodissociation of gas-phase I3−: Comprehensive understanding of nonadiabatic dissociation dynamics

Photodissociation of the gas-phase tri-iodide anion, I3-, was investigated using photofragment time of flight (TOF) mass spectrometry combined with the core extraction method. An analysis of the TOF profiles provided the kinetic energy and angular distributions of photofragment ions and photoneutrals, from which the photoproduct branching fractions were determined in the excitation energy range of 3.26-4.27 eV. The measurement has revealed that (1) in the entire energy range investigated, three-body dissociation occurs preferentially as the "charge-asymmetric" process I-(1S)+I(2P3/2)+I(2P3/2) with the yield of approximately 30%-40%, where the excess charge is localized on the end atoms of the dissociating I3-, and that (2) two-body dissociation via the 3Piu(0u+)<--1Sigmag+(0g+) excitation proceeds as I-(1S)+I2(X 1Sigmag+)/I2(A 3Pi1u) or I(2P3/2)+I2-(X 2Sigmau+) with the yield of approximately 60%, while that via the 1Sigmau+(0u+)<--1Sigmag+(0g+) excitation alternatively as I*(2P1/2)+I2-(X 2Sigmau+) or I-(1S)+I2(B 3Piu) with the yield of approximately 60%. Ab initio calculations including spin-orbit configuration interactions were also performed to gain precise information on the potential energy surfaces relevant to the I3- photodissociation. The calculations have shown the presence of conical intersections and avoided crossings located along the symmetric stretch coordinate near the ground-state equilibrium geometry of I3-, which play key roles for the two-body and the three-body product branching. The nonadiabatic nature of the I3- photodissociation dynamics is discussed by combining the experimental findings and the ab initio results.