Gas-phase electronic spectra of HCH ( = 2-6) chains.

Highly unsaturated carbon chains are generated in combustion processes and electrical discharges, and are confirmed constituents of the interstellar medium. In hydrogen-rich environments smaller carbon clusters tend to exist as linear chains, capped on each end by hydrogen atoms. Although the HCH polyacetylene chains have been extensively characterized spectroscopically, the corresponding odd HCH chains have received far less attention.

Bond dissociation energy of FeCr+ determined through threshold photodissociation in a cryogenic ion trap.

The bond dissociation energy of FeCr+ is measured using resonance enhanced photodissociation spectroscopy in a cryogenic ion trap. The onset for FeCr+ → Fe + Cr+ photodissociation occurs well above the lowest Cr+(6S, 3d5) + Fe(5D, 3d64s2) dissociation limit. In contrast, the higher energy FeCr+ → Fe+ + Cr photodissociation process exhibits an abrupt onset at the energy of the Cr(7S, 3d54s1) + Fe+(6D, 3d64s1) limit, enabling accurate dissociation energies to be extracted: D(Fe-Cr+) = 1.

Charge transfer transitions of the O2+-Ar and O2+-N2 complexes.

Electronic transitions are observed for the O2+-Ar and O2+-N2 complexes over the 225-350 nm range. The transitions are not associated with recognized electronic band systems of the respective atomic and diatomic constituents (Ar+, Ar, O2+, O2, N2+, and N2) but rather are due to charge transfer transitions. Onsets of the O2+-Ar and O2+-N2 band systems occur at 3.

Bond dissociation energies for Fe2+, Fe2O+, and Fe2O2+ clusters determined through threshold photodissociation in a cryogenic ion trap.

Understanding and controlling the chemical behavior of iron and iron oxide clusters requires accurate thermochemical data, which, because of the complex electronic structure of transition metal clusters, can be difficult to calculate reliably. Here, dissociation energies for Fe2+, Fe2O+, and Fe2O2+ are measured using resonance enhanced photodissociation of clusters contained in a cryogenically cooled ion trap. The photodissociation action spectrum of each species exhibits an abrupt onset for the production of Fe+ photofragments from which bond dissociation energies are deduced for Fe2+ (2.

Probing Colossal Carbon Rings.

Carbon aggregates containing between 10 and 30 atoms preferentially arrange themselves as planar rings. To learn more about this exotic allotrope of carbon, electronic spectra are measured for even cyclo[]carbon radical cations (-) using two-color photodissociation action spectroscopy. To eliminate spectral contributions from other isomers, the target cyclo[]carbon radical cations are isomer-selected using a drift tube ion mobility spectrometer prior to spectroscopic interrogation.

Excited-State Barrier Controls → Photoisomerization in -Hydroxycinnamate Biochromophores.

Molecules based on the deprotonated -hydroxycinnamate moiety are widespread in nature, including serving as UV filters in the leaves of plants and as the biochromophore in photoactive yellow protein. The photophysical behavior of these chromophores is centered around a rapid → photoisomerization by passage through a conical intersection seam. Here, we use photoisomerization and photodissociation action spectroscopies with deprotonated 4-hydroxybenzal acetone (CK) to characterize a wavelength-dependent bifurcation between electron autodetachment (spontaneous ejection of an electron from the S state because it is situated in the detachment continuum) and → photoisomerization.

Disentangling Electronic Spectra of Linear and Cyclic Hydrogenated Carbon Cluster Cations, CH ( = 3-10).

Electronic spectra are measured for protonated carbon clusters (CH) containing between 7 and 21 carbon atoms. Linear and cyclic CH isomers are separated and selected using a drift tube ion mobility stage before being mass selected and introduced into a cryogenically cooled ion trap. Spectra are measured using a two-color resonance-enhanced photodissociation strategy, monitoring C photofragments (H atom loss channel) as a function of excitation wavelength.

Photo-induced 6π-electrocyclisation and cycloreversion of isolated dithienylethene anions.

The diarylethene chromophore is commonly used in light-triggered molecular switches. The chromophore undergoes reversible 6π-electrocyclisation (ring closing) and cycloreversion (ring opening) reactions upon exposure to UV and visible light, respectively, providing bidirectional photoswitching. Here, we investigate the gas-phase photoisomerisation of - () and - () substituted dithienylethene carboxylate anions (DTE) using tandem ion mobility mass spectrometry coupled with laser excitation.

An ion mobility mass spectrometer coupled with a cryogenic ion trap for recording electronic spectra of charged, isomer-selected clusters.

Infrared and electronic spectra are indispensable for understanding the structural and energetic properties of charged molecules and clusters in the gas phase. However, the presence of isomers can potentially complicate the interpretation of spectra, even if the target molecules or clusters are mass-selected beforehand. Here, we describe an instrument for spectroscopically characterizing charged molecular clusters that have been selected according to both their isomeric form and their mass-to-charge ratio.

Photodissociation dynamics of N.

The photodissociation dynamics of N excited from its (linear) Σ /(bent) A″ ground to the first excited singlet and triplet states is investigated. Three-dimensional potential energy surfaces for the A', A″, and A' electronic states, correlating with the Δ and Π states in linear geometry, for N are constructed using high-level electronic structure calculations and represented as reproducing kernels. The reference ab initio energies are calculated at the MRCI+Q/aug-cc-pVTZ level of theory.

Electronic spectra of positively charged carbon clusters-C (n = 6-14).

Electronic spectra are measured for mass-selected C (n = 6-14) clusters over the visible and near-infrared spectral range through resonance enhanced photodissociation of clusters tagged with N molecules in a cryogenic ion trap. The carbon cluster cations are generated through laser ablation of a graphite disk and can be selected according to their collision cross section with He buffer gas and their mass prior to being trapped and spectroscopically probed. The data suggest that the C (n = 6-14) clusters have monocyclic structures with bicyclic structures becoming more prevalent for C and larger clusters.

Actinic Wavelength Action Spectroscopy of the IO Reaction Intermediate.

Iodinate anions are important in the chemistry of the atmosphere where they are implicated in ozone depletion and particle formation. The atmospheric chemistry of iodine is a complex overlay of neutral-neutral, ion-neutral, and photochemical processes, where many of the reactions and intermediates remain poorly characterized. This study targets the visible spectroscopy and photostability of the gas-phase hypoiodite anion (IO), the initial product of the I + O reaction, by mass spectrometry equipped with resonance-enhanced photodissociation and total ion-loss action spectroscopies.

Nonadiabatic Dynamics between Valence, Nonvalence, and Continuum Electronic States in a Heteropolycyclic Aromatic Hydrocarbon.

Internal conversion between valence-localized and dipole-bound states is thought to be a ubiquitous process in polar molecular anions, yet there is limited direct evidence. Here, photodetachment action spectroscopy and time-resolved photoelectron imaging with a heteropolycyclic aromatic hydrocarbon (hetero-PAH) anion, deprotonated 1-pyrenol, is used to demonstrate a subpicosecond (τ = 160 ± 20 fs) valence to dipole-bound state internal conversion following excitation of the origin transition of the first valence-localized excited state. The internal conversion dynamics are evident in the photoelectron spectra and in the photoelectron angular distributions (β values) as the electronic character of the excited state population changes from valence to nonvalence.

Photoisomerization of Linear and Stacked Isomers of a Charged Styryl Dye: A Tandem Ion Mobility Study.

The photoisomerization behavior of styryl 9M, a common dye used in material sciences, is investigated using tandem ion mobility spectrometry (IMS) coupled with laser spectroscopy. Styryl 9M has two alkene linkages, potentially allowing for four geometric isomers. IMS measurements demonstrate that at least three geometric isomers are generated using electrospray ionization with the most abundant forms assigned to a combination of (major) and (minor) geometric isomers, which are difficult to distinguish using IMS as they have similar collision cross sections.

Electronic Spectrum of the Tropylium Cation in the Gas Phase.

The structure and properties of the tropylium cation (CH) have enthralled chemists since the prediction by Hückel in 1931 of the remarkable stability for cyclic, aromatic molecules containing six π-electrons. However, probing and understanding the excited electronic states of the isolated tropylium cation have proved challenging, as the accessible electronic transitions are weak, and there are difficulties in creating appreciable populations of the tropylium cation in the gas phase. Here, we present the first gas-phase S ←S electronic spectrum of the tropylium cation, recorded by resonance-enhanced photodissociation of weakly bound tropylium-Ar complexes.

Photophysics of Isolated Rose Bengal Anions.

Dye molecules based on the xanthene moiety are widely used as fluorescent probes in bioimaging and technological applications due to their large absorption cross-section for visible light and high fluorescence quantum yield. These applications require a clear understanding of the dye's inherent photophysics and the effect of a condensed-phase environment. Here, the gas-phase photophysics of the rose bengal doubly deprotonated dianion [RB - 2H], deprotonated monoanion [RB - H], and doubly deprotonated radical anion [RB - 2H] is investigated using photodetachment, photoelectron, and dispersed fluorescence action spectroscopies, and tandem ion mobility spectrometry (IMS) coupled with laser excitation.

N : Full-dimensional ground state potential energy surface, vibrational energy levels, and dynamics.

The fundamental vibrational frequencies and higher vibrationally excited states for the N ion in its electronic ground state have been determined from quantum bound state calculations on three-dimensional potential energy surfaces (PESs) computed at the coupled-cluster singles and doubles with perturbative triples [CCSD(T)]-F12b/aug-cc-pVTZ-f12 and multireference configuration interaction singles and doubles with quadruples (MRCISD+Q)/aug-cc-pVTZ levels of theory. The vibrational fundamental frequencies are 1130 cm (ν, symmetric stretch), 807 cm (ν, asymmetric stretch), and 406 cm (ν, bend) on the higher-quality CCSD(T)-F12b surface. Bound state calculations based on even higher level PESs [CCSD(T)-F12b/aug-cc-pVQZ-f12 and MRCISD+Q-F12b/aug-cc-pVTZ-f12] confirm the symmetric stretch fundamental frequency as ∼1130 cm.

Photo- and Collision-Induced Isomerization of a Charge-Tagged Norbornadiene-Quadricyclane System.

Molecular photoswitches based on the norbornadiene-quadricylane (NBD-QC) couple have been proposed as key elements of molecular solar thermal energy storage schemes. To characterize the intrinsic properties of such systems, reversible isomerization of a charge-tagged NBD-QC carboxylate couple is investigated in a tandem ion mobility mass spectrometer, using light to induce intramolecular [2 + 2] cycloaddition of NBD carboxylate to form the QC carboxylate and driving the back reaction with molecular collisions. The NBD carboxylate photoisomerization action spectrum recorded by monitoring the QC carboxylate photoisomer extends from 290 to 360 nm with a maximum at 315 nm, and in the longer wavelength region resembles the NBD carboxylate absorption spectrum recorded in solution.

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

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

Reversible Photoswitching of Isolated Ionic Hemiindigos with Visible Light.

Indigoid chromophores have emerged as versatile molecular photoswitches, offering efficient reversible photoisomerization upon exposure to visible light. Here we report synthesis of a new class of permanently charged hemiindigos (HIs) and characterization of photochemical properties in gas phase and solution. Gas-phase studies, which involve exposing mobility-selected ions in a tandem ion mobility mass spectrometer to tunable wavelength laser radiation, demonstrate that the isolated HI ions are photochromic and can be reversibly photoswitched between Z and E isomers.

Electronic Spectra of Diacetylene Cations (HCH) Tagged with Ar and N.

Electronic spectra of mass-selected HCH-Ar ( = 1-3) and HCH-(N) ( = 1-2) complexes are measured over the 290-530 nm range using resonance-enhanced photodissociation spectroscopy in a tandem mass spectrometer. Vibronic transitions in the visible region are compared with previous experimental and theoretical results for the ÃΠ ← X̃Π band system of HCH. Hole burning experiments confirm that transitions over the 290-340 nm range involve the diacetylene cation (HCH).

Ultrafast photoisomerisation of an isolated retinoid.

The photoinduced excited state dynamics of gas-phase trans-retinoate (deprotonated trans-retinoic acid, trans-RA) are studied using tandem ion mobility spectrometry coupled with laser spectroscopy, and frequency-, angle- and time-resolved photoelectron imaging. Photoexcitation of the bright S(ππ*) ← S transition leads to internal conversion to the S(ππ*) state on a ≈80 fs timescale followed by recovery of S and concomitant isomerisation to give the 13-cis (major) and 9-cis (minor) photoisomers on a ≈180 fs timescale. The sub-200 fs stereoselective photoisomerisation parallels that for the retinal protonated Schiff base chromophore in bacteriorhodopsin.

Photodetachment and photoreactions of substituted naphthalene anions in a tandem ion mobility spectrometer.

Substituted naphthalene anions (deprotonated 2-naphthol and 6-hydroxy-2-naphthoic acid) are spectroscopically probed in a tandem drift tube ion mobility spectrometer (IMS). Target anions are selected according to their drift speed through nitrogen buffer gas in the first IMS stage before being exposed to a pulse of tunable light that induces either photodissociation or electron photodetachment, which is conveniently monitored by scavenging the detached electrons with trace SF6 in the buffer gas. The photodetachment action spectrum of the 2-naphtholate anion exhibits a band system spanning 380-460 nm with a prominent series of peaks spaced by 440 cm-1, commencing at 458.

Photoinitiated Intramolecular Proton Transfer in Deprotonated para-Coumaric Acid.

Deprotonated para-coumaric acid is commonly considered as a model for the chromophore in photoactive yellow protein, which undergoes E → Z isomerization following absorption of blue light. Here, tandem ion mobility mass spectrometry is coupled with laser excitation to study the photochemistry of deprotonated para-coumaric acid, to show that the E isomers of the phenoxide and carboxylate forms have distinct photochemical responses with maxima in their action spectra at 430 and 360 nm, respectively. The E isomer of the phenoxide anion undergoes efficient autodetachment upon excitation of its lowest ππ* transition.