Photofragmentation Spectra Research Articles

Vibrational and electronic spectra of protonated vanillin: exploring protonation sites and isomerisation.

Photofragmentation spectra of protonated vanillin produced under electrospray ionisation (ESI) conditions have been recorded in the 3000-3700 cm-1 (vibrational) and 225-460 nm (electronic) ranges, using room temperature IRMPD (infrared multiphoton dissociation) and cryogenic UVPD (ultraviolet photodissociation) spectroscopies, respectively. The cold (∼50 K) electronic UVPD spectrum exhibits very well resolved vibrational structure for the S1 ← S0 and S3 ← S0 transitions, suggesting long excited state dynamics, similar to its simplest analogue, protonated benzaldehyde. The experimental data were combined with theoretical calculations to determine the protonation site and configurational isomer observed in the experiments.

Excited-state chemistry of the nitromethane anion mediated by the dipole-bound states revealed by photofragment action spectroscopy.

We report the first experimental observation of the excited dipole-bound state (DBS) of the cryogenically cooled nitromethane anion (CH3NO2-), where the excess electron is loosely attached to the singlet or triplet neutral-core. Photofragment and photodetachment action spectra have been employed for the dynamic exploration of Feshbach resonances located even far above the electron detachment threshold, giving excitation profiles from the ground anionic state (D0) to the DBSs which match quite well with the spectral structures of the photoelectron spectra. This indicates that the electron transfer from the nonvalence orbital (of DBS) to the valence orbital (of anion) is mainly responsible for the anionic fragmentation channels, giving strong evidence for that the DBS plays a dynamic doorway-role in the anionic fragmentation reactions. Photofragment action spectra have also been obtained for the anionic clusters of (CH3NO2)2-, (CH3NO2)3-, or (CH3NO2·H2O)-, giving the relative yields of various fragments as a function of the excitation energy for each cluster. The absorption profiles of the anionic clusters exhibit substantial blue-shifts compared to the bare nitromethane anion as their ground states are much stabilized by solvation. The anionic fragmentation pattern varies among different clusters, giving essential clues for the thorough understanding of the whole anionic dynamics such as the dynamic role of the short-lived nonvalence-bound states of the clusters.

UV-VUV Photofragmentation Spectroscopy of Isolated Neutral Fragile Macromolecules: A Proof-of-Principle Based on a Deprotonated Vancomycin-Peptide Noncovalent Complex.

The gas phase offers the possibility to analyze organic molecules by ultraviolet-vacuum ultraviolet (UV-VUV) spectroscopy without any solvent effect or limitation in terms of spectral range due to absorption by the solvent. Up to now, the size and chemical composition of neutral molecular systems under study have been limited by the use of vaporization methods based on thermal heating. Soft sources of gas-phase thermolabile molecular systems such as electrospray or matrix-assisted laser desorption ionization are appealing alternatives to heating-based techniques, but they lead to the production of ions. In such cases, UV-VUV action spectroscopy is then the method of choice to study the electronic structure and corresponding photodynamics of these gas-phase molecular ions. However, previous investigations have shown that the UV-VUV action spectrum of a given molecular ion depends on the charge state, which in many cases might be a caveat. Here, by means of synchrotron radiation coupled to mass spectrometry and through the test case of the glycopeptide antibiotic vancomycin noncovalently bound to a deprotonated small peptide, we show that the UV-VUV photofragmentation spectrum of neutral thermally fragile organic molecules can be obtained via charge-tagging action spectroscopy.

Vacuum Ultraviolet Photodissociation Branching Ratios of 12C16O, 13C16O, and 12C18O from 100500 to 102320 cm-1.

The C+ ion photofragment spectra and photodissociation branching ratios into the two energetically available channels, C(1D) + O(3P) and C(3P) + O(3P), have been obtained for the three CO isotopologues, 12C16O, 13C16O, and 12C18O, in the vacuum ultraviolet range 100500-102320 cm-1. The two vibronic states of 1Σ+ symmetry, F(3dσ) 1Σ+(υ' = 1) and J(4sσ) 1Σ+(υ' = 0), predominantly dissociate into the lowest channel C(3P) + O(3P) through interactions with the repulsive D'1Σ+ state. All three vibronic states of 1Π symmetry, E'1Π(υ' = 1, 2) and G(3dπ) 1Π(υ' = 0), dissociate into both of the channels above. The photodissociation branching ratios into the channel C(1D) + O(3P) for E'1Π(υ' = 1, 2) are found to be independent of both the rotational quantum number and e/f parity, while those for G(3dπ) 1Π(υ' = 0) strongly depend on the rotational quantum number, indicating very different predissociation pathways between the valence states E'1Π(υ' = 1, 2) and the Rydberg state G(3dπ) 1Π(υ' = 0). The potential energy curves of CO in the aforementioned energy range and below have recently been well constructed due to a series of interplays between high-resolution spectroscopic studies and theoretical calculations; the photodissociation branching ratios measured in this study can provide further benchmarks for future theoretical investigations which aim to understand the detailed predissociation dynamics of CO.

Reinvestigation of the Rydberg W1Π(ν = 1) level of 12C16O, 13C16O, and 12C18O through rotationally dependent photodissociation branching ratio measurements

A recent high resolution photoabsorption study revealed that the Rydberg W1Π(ν = 1) level of carbon monoxide (CO) is perturbed by the valence E″1Π(ν = 0) level, and the predissociation linewidth shows drastic variation at the crossing point due to the interference effect [Heays et al., J. Chem. Phys. 141(14), 144311 (2014)]. Here, we reinvestigate the Rydberg W1Π(ν = 1) level for the three CO isotopologues, 12C16O, 13C16O, and 12C18O, by measuring the rotationally dependent photodissociation branching ratios. The C+ ion photofragment spectra obtained here reproduce the recent high resolution photoabsorption spectra very well, including the presence of the valence E″1Π(ν = 0) level. The photodissociation branching ratios into the spin-forbidden channel C(1D) + O(3P) show sudden increases at the crossing point between the W1Π(ν = 1) and E″1Π(ν = 0) levels, which is in perfect accordance with the drastic variation of the linewidth observed in the recent spectroscopic study. Further analysis reveals that the partial predissociation rate into the lowest channel C(3P) + O(3P) shows a much more prominent decrease at the crossing point, which is caused by the interference effect between the W1Π(ν = 1) and E″1Π(ν = 0) levels, than that into the spin-forbidden channel C(1D) + O(3P), and this is the reason of the sudden increase as observed in the photodissociation branching ratio measurements. We hope that the current experimental investigation will stimulate further theoretical studies, which could thoroughly address all the experimental observations in a quantitative way.

Photodissociation branching ratios for several absorption bands of 12C16O from 108,500 to 109,220 cm−1

In the present study, photodissociation branching ratios are measured with optimised experimental conditions for the 12C16O absorption bands with upper states of I′(5sσ) 1Σ+(v′ = 1), (6sσ) 1Σ+(v′ = 0) and (6pπ) 1Π(v′ = 0), which were not observed in the recent study covering the titled energy range (J. Chem. Phys. 137, 034305 (2012)). The absorption band with the upper state of (6pσ) 1Σ+(v′ = 0) has been reinvestigated, and a weak dependence of branching ratios into the spin-forbidden channels on the rotational quantum number is observed. In addition to these relatively strong absorption bands, two weak bands have also been identified in the C+ ion photofragment spectra, one with well rotationally resolved 1Π character and has never been observed before, and the other with diffuse structure and its upper state was assigned as C′1Σ+(v′ = 9) before in spectroscopic studies. Except for the two newly observed weak absorption bands, the four strong absorption bands all predominantly dissociate into the ground channel C(3P)+O(3P). By far, we have finished measuring the photodissociation branching ratios for all the strong 12C16O absorption bands of astrophysical importance above the photon energy of 102,700 cm−1.

Electron detachment dynamics of the iodide-guanine cluster: does ionization occur from the iodide or from guanine?

Laser photodissociation spectroscopy of the I‐·guanine complex has been conducted for the first time across the regions above the electron detachment threshold to explore the excited states and whether vertical ionisation occurs from the iodide or the nucleobase. The photofragment spectra reveal a prominent dipole-bound excited state (I) close to the calculated vertical electron detachment energy (∼4.0 eV) and a second excited (II) centred around 4.8 eV, which we assign to π-π* nucleobase-localised transitions. The ionic photofragments are identified as I‐ and I‐·[G-H], with the later fragment being produced significantly more strongly than the former. Both photofragments are observed across the two excited states, with production of the iodide being attributed to internal conversion to the ground state followed by evaporation. We trace the formation of the I‐·[G-H] photofragment to initial vertical ionisation of guanine, followed by ejection of a proton. This two-step process is important as it follows known steps in radiation-induced damage to DNA, namely initial formation of a guanine radical cation which then forms a free radical [G-H] moiety through deprotonation. Production of the I‐·[G-H] photofragment is pronounced through II indicating that its formation is enhanced by coupling of the π-π* transitions to the electron detachment continuum.

UV Photofragmentation of Cold Cytosine–M+ Complexes (M+: Na+, K+, Ag+)

The UV photofragmentation spectra of cold cytosine-M+ complexes (M+: Na+, K+, Ag+) were recorded and analyzed through comparison with geometry optimizations and frequency calculations of the ground and excited states at the SCS-CC2/Def2-SVPD level of theory. While in all complexes, the ground state minimum geometry is planar (Cs symmetry), the ππ* state minimum geometry has the NH2 group slightly twisted and an out-of-plane metal cation. This was confirmed by comparing the simulated ππ* Franck-Condon spectra with the vibrationally resolved photofragmentation spectra of CytNa+ and CytK+. Vertical excitation transitions were also calculated to evaluate the energies of the CT states involving the transfer of an electron from the Cyt moiety to M+. For both CytK+ and CytNa+ complexes, the first CT state corresponds to an electron transfer from the cytosine aromatic π ring to the antibonding σ* orbital centered on the alkali cation. This πσ* state is predicted to lie much higher in energy (>6 eV) than the band origin of the π-π* electronic transition (around 4.3 eV) unlike what is observed for the CytAg+ complex for which the first excited state has a nOσ* electronic configuration. This is the reason for the absence of the Cyt+ + M charge transfer fragmentation channel for CytK+ and CytNa+ complexes.

Photofragmentation of Bin+/- clusters (n = 2−19) in an electrostatic ion beam trap

Photofragmentation spectra for small bismuth-cluster cat- and anions are recorded for the investigation of their fragmentation pathways as a function of cluster size. To this end, the ions of interest are stored and size-selected in an electrostatic ion beam trap/a multi-reflection time-of-flight mass spectrometer with high resolving power. The subsequent photoexcitation is timed to their turn-around point in the trap’s mirror potential. In this novel approach, charged fragment clusters exhibit the same energy as the precursor ions and, thus, can be further stored and investigated with identical trap configurations. The results suggest a high stability of the neutral bismuth tetramer in accordance with previous reports, which is attributed to bismuth clusters exhibiting semimetal properties. Additionally, there is evidence for the loss of larger neutral fragments.

Peptide Bond Ultraviolet Absorption Enables Vibrational Cold-Ion Spectroscopy of Nonaromatic Peptides.

Peptide-bond VUV absorption is inherent to all proteins and peptides. Although widely exploited in top-down proteomics for photodissociation, this absorption has never been spectroscopically characterized in the gas phase. We have measured VUV/UV photofragmentation spectrum of a single peptide bond in a cryogenically cold protonated dipeptide. Although the spectrum appears to be very broadband and structureless, vibrational pre-excitation of this and even larger cold peptides significantly increases the UV dissociation yield for some of their photofragments. We use this effect to extend the technique of IR-UV photofragmentation vibrational spectroscopy, developed for aromatic peptides, to nonaromatic ones and demonstrate measurements of conformation-specific and nonspecific IR spectra for di- to hexa-peptides.

Photodissociation of Free Metalloporphyrin Dimer Multianions.

We have used action photofragmentation spectroscopy in the visible spectral range (410 to 650 nm) to investigate the optical properties of different monomeric and dimeric M(II)-meso-tetra-(4-sulfonatophenyl)-porphyrin (with M = Pd(II), Cu(II), Zn(II)) multianions isolated in the gas phase without solvent. In particular, we report the position of the Q-bands (S0 → S1 transitions) as a function of charge state, counterions, oligomerization, and dimer structure type. The results for the monomers (charge states = 4- and 3-, sodiated and protonated) are in good agreement with TDDFT calculations and condensed-phase spectra. For both homo and heterometallic dimers, photofragmentation spectra were recorded for two charge states, 5- and 3-, corresponding to coplanar and cofacial structure types, respectively. The fragmentation patterns observed for the dimers depend significantly on charge state, with fragmentation into monomers being dominant for the 5- species, while the 3- charge state predominantly fragments by SO2 loss. The monomer → dimer Q-band spectral shifts observed in the gas phase were compared with the optical properties of porphyrin aggregates in solution.

Watching proton transfer in real time: Ultrafast photoionization-induced proton transfer in phenol-ammonia complex cation.

In this paper, we give a full account of our previous work [C. C. Shen et al., J. Chem. Phys. 141, 171103 (2014)] on the study of an ultrafast photoionization-induced proton transfer (PT) reaction in the phenol-ammonia (PhOH-NH3) complex using ultrafast time-resolved ion photofragmentation spectroscopy implemented by the photoionization-photofragmentation pump-probe detection scheme. Neutral PhOH-NH3 complexes prepared in a free jet are photoionized by femtosecond 1 + 1 resonance-enhanced multiphoton ionization via the S1 state. The evolving cations are then probed by delayed pulses that result in ion fragmentation, and the ionic dynamics is followed by measuring the parent-ion depletion as a function of the pump-probe delay time. By comparing with systems in which PT is not feasible and the steady-state ion photofragmentation spectra, we concluded that the observed temporal evolutions of the transient ion photofragmentation spectra are consistent with an intracomplex PT reaction after photoionization from the initial non-PT to the final PT structures. Our experiments revealed that PT in [PhOH-NH3]+ cation proceeds in two distinct steps: an initial impulsive wave-packet motion in ∼70 fs followed by a slower relaxation of about 1 ps that stabilizes the system into the final PT configuration. These results indicate that for a barrierless PT system, even though the initial PT motions are impulsive and ultrafast, the time scale to complete the reaction can be much slower and is determined by the rate of energy dissipation into other modes.

Vibronic spectra of protonated hydroxypyridines: contributions of prefulvenic and planar structures.

Various hydroxypyridine derivatives are endogenous or synthetic photosensitizers which could contribute to solar radiation damage. The study of their excited states could lead to a better understanding of their action mechanisms. We present here the ultraviolet (UV) spectra of the protonated 2-, 3- and 4-hydroxypyridine. These spectra were obtained with an experimental device coupling an electrospray ion source with a cold quadrupole ion trap and a time of flight mass spectrometer. They display well resolved vibrational structures, with a clear influence of the position of the OH group. These results are interpreted with excited states calculations at the coupled cluster CC2 level.

Excited States of Xanthene Analogues: Photofragmentation and Calculations by CC2 and Time‐Dependent Density Functional Theory

AbstractThe front cover artwork is provided by the SpectroBio group at the Institut Lumière Matière in Lyon. The image shows the composition of our experimental–theoretical work: a photo of our nanosecond laser setup in the background with the laser‐pulse dependent photofragmentation spectra is the central element. Further elements are the Jacobian matrix for calculating CC2 transition energies, a natural transition orbital, and the energy levels and processes used for constructing the photofragmentation model. Read the full text of the article at 10.1002/cphc.201600650.

Excited States of Xanthene Analogues: Photofragmentation and Calculations by CC2 and Time-Dependent Density Functional Theory.

Action spectroscopy has emerged as an analytical tool to probe excited states in the gas phase. Although comparison of gas-phase absorption properties with quantum-chemical calculations is, in principle, straightforward, popular methods often fail to describe many molecules of interest-such as xanthene analogues. We, therefore, face their nano- and picosecond laser-induced photofragmentation with excited-state computations by using the CC2 method and time-dependent density functional theory (TDDFT). Whereas the extracted absorption maxima agree with CC2 predictions, the TDDFT excitation energies are blueshifted. Lowering the amount of Hartree-Fock exchange in the DFT functional can reduce this shift but at the cost of changing the nature of the excited state. Additional bandwidth observed in the photofragmentation spectra is rationalized in terms of multiphoton processes. Observed fragmentation from higher-lying excited states conforms to intense excited-to-excited state transitions calculated with CC2. The CC2 method is thus suitable for the comparison with photofragmentation in xanthene analogues.

Excited state of protonated benzene and toluene.

We present photo-fragmentation electronic spectra of the simplest protonated aromatic molecules, protonated benzene and toluene, recorded under medium resolution conditions and compared with the photo-fragmentation spectrum of protonated pyridine. Despite the resolution and cold temperature achieved in the experiment, the electronic spectra of protonated benzene and toluene are structure-less, thus intrinsically broadened. This is in agreement with the large geometrical changes and the fast dynamic toward internal conversion predicted by ab initio calculations for protonated benzene [Rode et al., J. Phys. Chem. A 113, 5865-5873 (2009)].

Communication: Ultrafast time-resolved ion photofragmentation spectroscopy of photoionization-induced proton transfer in phenol-ammonia complex

Photoionization-induced proton transfer (PT) in phenol-ammonia (PhOH-NH3) complex has been studied using ultrafast time-resolved ion photofragmentation spectroscopy. Neutral PhOH-NH3 complexes prepared in a free jet are photoionized by femtosecond [1+1] resonance-enhanced multiphoton ionization via the S1 state, and the subsequent dynamics occurring in the cations is probed by delayed pulses that result in ion fragmentation. The observed temporal evolutions of the photofragmentation spectra are consistent with an intracomplex PT reaction. The experiments revealed that PT in [PhOH-NH3](+) cation proceeds in two distinct steps: an initial impulsive wave-packet motion in ~70 fs followed by a slower relaxation of about 1 ps that stabilizes the system into the final PT configuration. These results indicate that for a barrierless PT system, even though the initial PT motions are impulsive and ultrafast, the reaction may take a much longer time scale to complete.

Communication: identification of daughter ions through their electronic spectroscopy at low temperature.

We present experimental results on photofragmentation of cooled fragments issued from the photofragmentation of cold parent ions. The cooling of the daughter ions at a few K allows its characterization not only through its fragmentation pattern but also through its well resolved electronic spectroscopy. The proof of principle of the method is demonstrated on the photofragment resulting from Cα-Cβ bond rupture in protonated tyrosine (TyrH(+)). The analysis of the daughter ion (m/z 108) photofragmentation spectrum is in agreement with the proposed mechanism implying a proton transfer to the phenyl ring as the first step of the fragmentation mechanism of TyrH(+).

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.

Excited states of proton-bound DNA/RNA base homodimers: pyrimidines.

We are presenting the electronic photofragment spectra of the protonated pyrimidine DNA base homodimers. Only the thymine dimer exhibits a well structured vibrational progression, while the protonated monomer shows broad vibrational bands. This shows that proton bonding can block some nonradiative processes present in the monomer.