Photodissociation Spectra Research Articles

Intermolecular Interactions between Aromatic Amino Acids Investigated by Ultraviolet Photodissociation Spectroscopy of Hydrogen-Bonded Clusters of Histidine and Tryptophan Enantiomers.

Intermolecular interactions between aromatic amino acids were investigated by ultraviolet photodissociation spectroscopy of hydrogen-bonded protonated clusters of histidine (His) and tryptophan (Trp) enantiomers in the gas phase. Product ion spectra and photodissociation spectra in the wavelength range of the S1-S0 transition of Trp at several temperatures (8-100 K) were obtained using a tandem mass spectrometer equipped with an electrospray ionization source and a cold ion trap. l-Trp detachment forming protonated His was the main pathway. Two bands observed at 288 and 285 nm in the photodissociation spectra of heterochiral H+(d-His)(l-Trp) indicated the coexistence of two types of conformers. The bands at 288 and 285 nm were attributed to the conformers formed from stronger and weaker intermolecular interactions, respectively. In the spectra of homochiral H+(l-His)(l-Trp), only the band due to the stronger interactions was observed at 288 nm. The intermolecular interactions of l-His with l-Trp were stronger than those of d-His with l-Trp.

Enantioselective complexation of protonated tyrosine by a chiral crown-ether: the nature of the Hydrogen bonds makes the difference.

The complexes formed between (18-crown-6)-tetracarboxylic acid, denoted as 18C6TA, and the two enantiomers of protonated tyrosine, L- and D-Tyr, are studied in a cryogenic ion trap by combining mass spectrometry, laser spectroscopy, and DFT calculations. Both UV and IR photodissociation spectra indicate the formation of multiple isomers for each complex, some of them interconverting upon IR irradiation. Conformer-selective vibrational spectroscopy reveals that all structures involve an internally hydrogen-bonded folded structure of the crown ether. The complexes formed with L-Tyr involve two NH…O interactions with the ether oxygen atoms,and two hydrogen bonds to the crown ether carboxylic function, one from the NH group and the other from the COOH group of tyrosine. TheD-Tyr complexesshow more conformational mobility: two out of the three lowest energy conformers observed are tripodal, with three NH…O interactions between the amino acid ammonium and the crown ether oxygens. An additional conformer shows only one NH…O interaction, but has two interactions involving one of the cavity COOH moieties, one with the NH and one with the phenol OH. The increased calculated stability of the complex made from (-) 18C6TA and L-Tyr parallels its higher abundance in the mass spectrum of an isotopically labelled racemic mixture.

Study on photodissociation spectra and decay pathways of gas-phase PdCl3– and PdCl2– anions by electrospray ionization mass spectrometry and MRCI calculation

We investigate photodissociation of the gas-phase palladium complex anions, Pd(II)Cl3- and Pd(I)Cl2-, both experimentally and theoretically. Visible photodissociation mass spectrometry reveals that the main dissociation channels differ for Pd(II)Cl3- and Pd(I)Cl2-; the former yields neutral chlorine atom (Pd(I)Cl2- + Cl) whereas the latter affords chloride anion (Pd(I)Cl + Cl-). Absorption spectrum and photodissociation channels are predicted using the MRCI method. The observed absorption spectrum and dissociation product for Pd(II)Cl3- agree with the MRCI calculation, whereas those for Pd(I)Cl2- do not, which suggests that some relaxation process such as internal conversion significantly contributes to the photodissociation of Pd(I)Cl2-.

IR and UV Spectroscopy of Gas-Phase Monohydrated Protonated Guanine.

We use UV and infrared photodissociation spectroscopy to study monohydrated protonated guanine in a dual cryogenic ion trap spectrometer. The monohydrated complexes are formed through helium-mediated collisions between bare electrosprayed protonated guanine and low-pressure water vapor in a clustering trap maintained at 180 K, before being transferred to a quadrupole ion trap at 10 K. The spectrum of the monohydrated complex exhibits sharp vibronic transitions at the band origin and becomes broader and higher in intensity further in blue, which is very similar to protonated guanine but with a notable blue shift of ∼1850 cm-1 (∼0.23 eV). The UV hole-burning experiments showed that the vibronic bands recorded in the region of the band origin belong to a single conformer under our experimental conditions. The IR photodissociation spectrum in the 3000-3600 cm-1 range, with the aid of theoretical calculations (SCS-CC2/aug-cc-pVDZ), allowed us to assign the structure to the lowest energy N7-O conformer.

Conformation and Photodissociation Process of Benzo-15-Crown-5 and Benzo-18-Crown-6 Complexes with Ammonium Ions Investigated by Cold UV and IR Spectroscopy in the Gas Phase.

We examined the conformation of benzo-15-crown-5 (B15C5) and benzo-18-crown-6 (B18C6) complexes with ammonium ions, NH4+, CH3NH3+ (MeNH3+), CH3CH2NH3+ (EtNH3+), and CH3CH2CH2NH3+ (PrNH3+), using cold UV and IR spectroscopy in the gas phase. We measured the UV photodissociation (UVPD) spectra of the ammonium complexes and compared them with those of the K+(B15C5) and K+(B18C6) complexes in order to identify the conformation on the basis of the band position. The number of possible conformations for the ammonium complexes of B15C5 is limited compared with alkali metal ions with similar ionic radii. The NH4+(B15C5), MeNH3+(B15C5), and EtNH3+(B15C5) complexes show two conformers, whereas the K+(B15C5) complex has three stable conformers. In the case of the PrNH3+(B15C5) complex, one conformer was found predominantly in the UVPD spectrum. The ammonium complexes of B15C5 prefer to adopt crown conformations with large dihedral angles on the C-O-C-C atoms around the benzene moiety. In the case of the ammonium complexes of B18C6, two or three conformers were found in the UVPD spectra. One conformation of the B18C6 complexes is similar to that of the K+(B18C6) complex, which has a planar form on the C-O-C-C atoms around the benzene moiety. The other but dominant conformations of the ammonium complexes could be attributed to those with large C-O-C-C dihedral angles. These conformational findings for the ammonium complexes suggest that the benzo-crown ethers tend to adopt nonplanar conformations around the benzene moiety to encapsulate the ammonium ions. The IR-UV double-resonance (DR) spectra of the B15C5 and B18C6 complexes were compared to those of benzo-12-crown-4 (B12C4) and dibenzo-18-crown-6 (DB18C6) complexes. The N-H···O hydrogen bond (H-bond) is weaker with increasing ring size from B12C4 to B18C6, although the calculated binding energy is smaller for B12C4 than for B18C6. This result indicates that cooperative H-bonds with three N-H groups can strengthen the intermolecular bond between the ammonium ions and B18C6. The difference in the conformational preference between the ammonium and K+ complexes is attributed to directed N-H···O H-bonds in the ammonium complexes. Proton transfer and dissociation of the crown ring were also observed for the photoexcitation of the NH4+(B15C5) and NH4+(B18C6) complexes.

Cryogenic Ion Vibrational Spectroscopy of Protonated Valine: Messenger Tag Effects.

We report the infrared photodissociation spectrum of tagged protonated valine in the range 1000-1900 cm-1, prepared in a cryogenic ion trap. Comparison of experimental results with calculated infrared spectra based on density functional theory shows that the hydroxyl group of the carboxylic acid functionality and the protonated amine group adopt a trans configuration. Nitrogen and methane molecules were used as messenger tags with optimal tagging temperatures of 30 K for N2 and 60 K for CH4. While the calculated infrared spectra of the tagged ion suggest only a weak influence of the messenger tag on the frequency positions of ValH+, the measured intensities for N2-tagged ValH+ appear strongly suppressed for all but the highest frequency feature at 1773 cm-1. We trace this behavior to the binding energy of the N2 tag, which is significantly higher than that of CH4, based on density functional and coupled cluster calculations and rate estimates for photoinduced unimolecular dissociation from statistical theory.

Observation of the Infrared-Induced Structural Change in the Microscopic Hydrogen Bond Network of Phenol-Methanol Cluster Cations in a Cold-Ion Trap.

To gain insight into microscopic hydrogen bond networks, we measured ultraviolet photodissociation (UVPD) spectra of the phenol-methanol 1:3 cluster cation, [PhOH(MeOH)3]+ trapped in a variable temperature ion trap. At low temperatures, an isomer with a ring-type hydrogen bond structure dominates, whereas at higher temperatures the chain-type isomers become dominant due to the flexibility of their hydrogen bond structures. We also found a clear temperature dependence of the spectral features, such as band position and width. In addition to the above measurement, we observed the infrared (IR) induced isomerization of [PhOH(MeOH)3]+ to study the dynamical aspects of hydrogen bond networks. We succeeded in observing IR-induced isomerization from the ring to chain forms of [PhOH(MeOH)3]+ at low temperature. The isomerization was clearly identified as a change in the UVPD spectra. The time evolution of the UVPD spectra after IR excitation indicated that the IR-induced isomerization occurs within a nanosecond. The chain-type isomers produced by the IR-induced isomerization are then converted back to the ring-type form by collisions with cold He buffer gas in the trap. This backward isomerization proceeds with a time constant of 100 μs under our experimental conditions. In this study, we evaluated the temperatures of the chain isomers during the backward isomerization on the basis of the spectral features.

Multiconfigurational Character of Repulsive A2Σg+ State Leaves Strong Signature in the Photodissociation Spectrum of Zn2.

For the excitation to a repulsive state of a diatomic molecule, one expects a single broad peak in the photodissociation spectrum. For Zn2+, however, two peaks for the spin- and symmetry-allowed A2Σg+ ← X2Σu+ transition are observed. A detailed quantum-chemical analysis reveals pronounced multiconfigurational character of the A2Σg+ state. The σg(4s)2σg(4p) configuration with bond order 1.5 dominates at short distances, while the repulsive σg(4s)σu*(4s)2 configuration with bond order -0.5 wins over with increasing bond length. The two excited-state configurations contribute with opposite signs to the transition dipole moment, which reaches zero near the equilibrium distance. This local minimum of the oscillator strength is responsible for the pronounced dip in the photodissociation spectrum, which is thus the spectroscopic signature of the multiconfigurational character of the A2Σg+ state.

Quantum study of the CH3+ photodissociation in full-dimensional neural network potential energy surfaces.

C H 3 + , a cornerstone intermediate in interstellar chemistry, has recently been detected for the first time by using the James Webb Space Telescope. The photodissociation of this ion is studied here. Accurate explicitly correlated multi-reference configuration interaction abinitio calculations are done, and full-dimensional potential energy surfaces are developed for the three lower electronic states, with a fundamental invariant neural network method. The photodissociation cross section is calculated using a full-dimensional quantum wave packet method in heliocentric Radau coordinates. The wave packet is represented in angular and radial grids, allowing us to reduce the number of points physically accessible, requiring to push up the spurious states appearing when evaluating the angular kinetic terms, through projection technique. The photodissociation spectra, when employed in astrochemical models to simulate the conditions of the Orion bar, result in a lesser destruction of CH3+ compared to that obtained when utilizing the recommended values in the kinetic database for astrochemistry.

Iron Complexes as Potential Carriers of Diffuse Interstellar Bands: The Photodissociation Spectrum of Fe+(H2O) at Optical Wavelengths.

The fullerene ion C60+ is the only carrier of diffuse interstellar bands (DIBs) identified so far. Transition-metal compounds feature electronic transitions in the visible and near-infrared regions, making them potential DIB carriers. Since iron is the most abundant transition metal in the cosmos, we here test this idea with Fe+(H2O). Laboratory spectra were obtained by photodissociation spectroscopy at 80 K. Spectra were modeled with the reflection principle. A high-resolution spectrum of the DIB standard star HD 183143 served as an observational reference. Two broad bands were observed from 4120 to 6800 Å. The 4120-4800 Å band has sharp features emerging from the background, which have the width of DIBs but do not match the band positions of the reference spectrum. Calculations show that the spectrum arises from a d-d transition at the iron center. While no match was found for Fe+(H2O) with known DIBs, the observation of structured bands with line widths typical for DIBs shows that small molecules or molecular ions containing iron are promising candidates for DIB carriers.

A time-independent, variational method for studying the photodissociation of triatomic molecules.

The photodissociation of molecules is becoming an increasingly important factor to consider in the evolution of exoplanets' atmospheres orbiting around UV-rich stars, as it leads to the enrichment of atmospheric complexity. A new method is developed for computing the rotationally and vibrationally resolved photodissociation spectrum of triatomic molecules. The time-independent Schrödinger equation is solved using the variational nuclear motion program EVEREST; a new code EXOCSMOOTH is employed to compute the cross-sections by applying Gaussian smoothing to a set of discrete transitions into the continuum. HCN is chosen as the test molecule, as it has been widely studied in the literature. Results are compared with the available experiments. Temperature dependence is explored for temperatures up to 2000 K.

D-Amino acid recognition of tripeptides studied by ultraviolet photodissociation spectroscopy of hydrogen-bonded clusters.

To understand the roles of D-amino acids, evaluating their chemical properties in living organisms is essential. Herein, D-amino acid recognition of peptides was investigated using a tandem mass spectrometer equipped with an electrospray ionization source and a cold ion trap. Ultraviolet (UV) photodissociation spectroscopy and water adsorption of hydrogen-bonded protonated clusters of tryptophan (Trp) enantiomers and tripeptides (SAA, ASA, and AAS, where S and A denote L-serine and L-alanine, respectively) were carried out at 8K in the gas phase. In the UV photodissociation spectrum of H+(D-Trp)ASA, the bandwidth of the S1-S0 transition, which corresponds to the ππ* state of the Trp indole ring, was narrower than those of the other five clusters, H+(D-Trp)SAA, H+(D-Trp)AAS, H+(L-Trp)SAA, H+(L-Trp)ASA, and H+(L-Trp)AAS. In the UV photoexcitation of H+(D-Trp)ASA(H2O)n, which were formed via water adsorption on gas-phase H+(D-Trp)ASA, the evaporation of water molecules was the main photodissociation pathway. An NH2CHCOOH-eliminated ion and H+ASA were observed in the product ion spectrum. By contrast, water molecules adsorbed on the other five clusters remained on the product ions for NH2CHCOOH elimination and Trp detachment after the UV photoexcitation. The results indicated that the indole ring of Trp was located on the surface of H+(D-Trp)ASA, and the amino and carboxyl groups of Trp formed hydrogen bonds in H+(D-Trp)ASA. For the other five clusters, the indole rings of Trp were hydrogen bonded in the clusters, and the amino and carboxyl groups of Trp were present on the cluster surfaces.

Electronic Interaction between Aromatic Chromophores in Dibenzo-Crown Ether Complexes with Alkali Metal Ions Investigated via Cold Gas-Phase Spectroscopy

This study investigated the geometric and electronic structures of dibenzo-21-crown-7 (DB21C7) and dibenzo-24-crown-8 (DB24C8) complexes with alkali metal ions, identified as M+(DB21C7) and M+(DB24C8) (M = Na, K, Rb, and Cs), respectively. We observed the ultraviolet photodissociation (UVPD) spectra of these complexes under cold (∼10 K) gas-phase conditions. The conformations of the M+(DB21C7) and M+(DB24C8) complexes were determined by comparing the UVPD spectra with the calculated electronic transitions of the local-minimum forms. The interactions between the electronic excited states of the two benzene chromophores in the M+(DB21C7) and M+(DB24C8) complexes were examined and compared with those of previously studied complexes (dibenzo-15-crown-5 (DB15C5) and dibenzo-18-crown-6 (DB18C6)). The S1-S0 and S2-S0 electronic excitations of the M+(DB21C7) complexes were almost localized in one of the benzene rings. In contrast, the closed conformers of the M+(DB24C8) (M = K, Rb, and Cs) complexes were delocalized over the two chromophores for electronic excitations, exhibiting strong electronic interactions between the benzene rings. For the M+(DB24C8) complexes (M = K, Rb, and Cs), the short distance between the benzene rings (∼3.9 Å) led to a strong interaction between the benzene chromophores. We conclude that this strong interaction in the M+(DB24C8) complexes correlates strongly with the broad absorption in the UVPD spectra, suggesting the presence of an intramolecular excimer for the K+(DB24C8), Rb+(DB24C8), and Cs+(DB24C8) complexes.

Copper(II)‐TEMPO Interaction

AbstractCopper(II) complexes with N‐oxyl reactants such as TEMPO can selectively oxidize alcohols to aldehydes or ketones. The proposed copper intermediates of the oxidation reaction were extensively theoretically studied, but they were never experimentally detected. Here, we present an analysis of “frozen” intermediates that contain alcohols without α‐hydrogen atoms, thus preventing oxidation. The copper(II)‐TEMPO complexes with a bipyridine‐type ancillary ligand were isolated by electrospray ionization mass spectrometry and investigated spectroscopically by cryogenic photodissociation spectroscopy. The vibrational characteristics of the complexes suggest that TEMPO retains its unpaired electron even upon coordination with copper(II). In agreement, the electronic photodissociation spectra of the TEMPO‐copper(II) complexes show a characteristic band for gaseous copper(II) complexes. These results contradict interpretations of some previous density functional theory (DFT) analyses of possible reaction intermediates. The experimental data were confronted with theoretical results obtained by pure DFT (OPBE, TPSS) and hybrid DFT (TPSSH, B3LYP) calculations. The methods favor different ground states and capture various aspects of the experimental results demonstrating a multiconfigurational character of the copper(II)‐TEMPO complexes.

Observation of Slow Eigen-Zundel Interconversion in H+(H2O)6 Clusters upon Isomer-Selective Vibrational Excitation and Buffer Gas Cooling in a Cryogenic Ion Trap.

The formation of isomers when trapping floppy cluster ions in a temperature-controlled ion trap is a generally observed phenomenon. This involves collisional quenching of the ions initially formed at high temperature by buffer gas cooling until their internal energies fall below the barriers in the potential energy surface that separate them. Here we explore the kinetics at play in the case of the two isomers adopted by the H+(H2O)6 cluster ion that differ in the proton accommodation motif. One of these is most like the Eigen cation with a tricoordinated hydronium motif (denoted E), and the other is most like the Zundel ion with the proton equally shared between two water molecules (denoted Z). After initial cooling to about 20 K in the radiofrequency (Paul) trap, the relative populations of these two spectroscopically distinct isomers are abruptly changed through isomer-selective photoexcitation of bands in the OH stretching region with a pulsed (∼6 ns) infrared laser while the ions are in the trap. We then monitor the relaxation of the vibrationally excited clusters and reformation of the two cold isomers by recording infrared photodissociation spectra with a second IR laser as a function of delay time from the initial excitation. The latter spectra are obtained after ejecting the trapped ions into a time-of-flight photofragmentation mass spectrometer, thus enabling long (∼0.1 s) delay times. Excitation of the Z isomer is observed to display long-lived vibrationally excited states that are collisionally cooled on a ms time scale, some of which quench into the E isomer. These excited E species then display spontaneous interconversion to the Z form on a ∼10 ms time scale. These qualitative observations set the stage for a series of experimental measurements that can provide quantitative benchmarks for theoretical simulations of cluster dynamics and the potential energy surfaces that underlie them.

Cryogenic Ion Spectroscopy of Protonated and Sodiated Methyladenine Derivatives.

Ultraviolet photodissociation (UVPD) spectra of protonated 9-methyladenine (H+9MA), protonated 7-methyl adenine (H+7MA), protonated 3-methyladenine (H+3MA), and sodiated 7-methyladenine (Na+7MA) near the origin bands of the S0-S1 transition were obtained using cryogenic ion spectroscopy. The UV-UV hole burning, infrared (IR) ion-dip, and IR-UV double resonance spectra showed that all the ions were present as single isomers in a cryogenic ion trap. The UVPD spectrum of H+9MA exhibited only a broad absorption band, whereas the spectra of H+7MA, H+3MA, and Na+7MA displayed moderately or well-resolved vibronic bands. Potential energy profiles were computed to understand the reason for the different bandwidths of the vibronic bands in the spectra. The broadening of the bands was correlated with the slopes between the Franck-Condon point and the conical intersection between the S1 and S0 states in the potential energy profiles, thus reflecting the deactivation rates in the S1 state.

Photofragment imaging differentiates between one- and two-photon dissociation pathways in MgI.

The bond strength and photodissociation dynamics of MgI+ are determined by a combination of theory, photodissociation spectroscopy, and photofragment velocity map imaging. From 17 000 to 21 500cm-1, the photodissociation spectrum of MgI+ is broad and unstructured; photofragment images in this region show perpendicular anisotropy, which is consistent with absorption to the repulsive wall of the (1) Ω = 1 or (2) Ω = 1 states followed by direct dissociation to ground state products Mg+ (2S) + I (2P3/2). Analysis of photofragment images taken at photon energies near the threshold gives a bond dissociation energy D0(Mg+-I) = 203.0 ± 1.8 kJ/mol (2.10 ± 0.02eV; 17 000 ± 150cm-1). At photon energies of 33 000-41 000cm-1, exclusively I+ fragments are formed. Over most of this region, the formation of I+ is not energetically allowed via one-photon absorption from the ground state of MgI+. Images show the observed product is due to resonance enhanced two-photon dissociation. The photodissociation spectrum from 33 000 to 38 500cm-1 shows vibrational structure, giving an average excited state vibrational spacing of 227cm-1. This is consistent with absorption to the (3) Ω = 0+ state from ν = 0, 1 of the (1) Ω = 0+ ground state; from the (3) Ω = 0+ state, absorption of a second photon results in dissociation to Mg* (3P° J) + I+ (3PJ). From 38 500 to 41 000cm-1, the spectrum is broad and unstructured. We attribute this region of the spectrum to one-photon dissociation of vibrationally hot MgI+ at low energy and ground state MgI+ at higher energy to form Mg (1S) + I+ (3PJ) products.

Quantum chemistry simulation of ground- and excited-state properties of the sulfonium cation on a superconducting quantum processor.

The computational description of correlated electronic structure, and particularly of excited states of many-electron systems, is an anticipated application for quantum devices. An important ramification is to determine the dominant molecular fragmentation pathways in photo-dissociation experiments of light-sensitive compounds, like sulfonium-based photo-acid generators used in photolithography. Here we simulate the static and dynamical electronic structure of the H3S+ molecule, taken as a minimal model of a triply-bonded sulfur cation, on a superconducting quantum processor of the IBM Falcon architecture. To this end, we generalize a qubit reduction technique termed entanglement forging or EF [A. Eddins et al., Phys. Rev. X Quantum, 2022, 3, 010309], currently restricted to the evaluation of ground-state energies, to the treatment of molecular properties. While in a conventional quantum simulation a qubit represents a spin-orbital, within EF a qubit represents a spatial orbital, reducing the number of required qubits by half. We combine the generalized EF with quantum subspace expansion [W. Colless et al., Phys. Rev. X, 2018, 8, 011021], a technique used to project the time-independent Schrodinger equation for ground- and excited-states in a subspace. To enable experimental demonstration of this algorithmic workflow, we deploy a sequence of error-mitigation techniques. We compute dipole structure factors and partial atomic charges along ground- and excited-state potential energy curves, revealing the occurrence of homo- and heterolytic fragmentation. This study is an important step towards the computational description of photo-dissociation on near-term quantum devices, as it can be generalized to other photodissociation processes and naturally extended in different ways to achieve more realistic simulations.

Carbon dioxide activation by discandium dioxide cations in the gas phase: a combined investigation of infrared photodissociation spectroscopy and DFT calculations.

We present a combined computational and experimental study of CO2 activation at the Sc2O2+ metal oxide ion center in the gas phase. Density functional theory calculations on the structures of [Sc2O2(CO2)n]+ (n = 1-4) ion-molecule complexes reveal a typical end-on binding motif as well as bidentate and tridentate carbonate-containing configurations. As the number of attached CO2 molecules increases, activated forms tend to dominate the isomeric populations. Distortion energies are unveiled to account for the conversion barriers from molecularly bound isomers to carbonate structures, and show a monotonically decreasing trend with successive CO2 ligand addition. The infrared photodissociation spectra of target ion-molecule complexes were recorded in the 2100-2500 cm-1 frequency region and interpreted by comparison with simulated IR spectra of low-lying isomers representing distinct configurations, demonstrating a high possibility of carbonate structure formation in current experiments.

Ultraviolet photoabsorption in the B 3Σ- − X 3Σ- and C 3Π − X 3Σ- band systems of SO sulphur isotopologues

High-resolution far-ultraviolet broadband Fourier-transform photoabsorption spectra of 32S16O, 33S16O, 34S16O, and 36S16O are recorded in a microwave discharge seeded with SO2. The and bands are observed or inferred in the –51,000 cm−1 (196 to 233 nm) spectral range. This is the first experimental detection of a level and of any of these observed bands in an S-substituted isotopologue. Additional measurements of provide a calibration of the SO column density. Measured band profiles are fitted to an effective-Hamiltonian model of coupled excited B 3Σ− and C 3Π states along with their predissociation linewidths and absorption band strengths. Electronic-state potential-energy curves and transition moments are deduced. The end result is a list of line frequencies, f-values, and dissociation widths describing the far-ultraviolet photodissociation spectrum of SO that is accurate enough for computing atmospheric photolytic isotope-fractionation. Linelists, and photoabsorption and photodissociation cross sections are available as supplementary data.