Photodissociation Products Research Articles

Duality of roaming mechanism in H2CO

Chemical Physics The phenomenon of roaming in chemical reactions (that is, bypassing the minimum energy pathway from unlikely geometries) has attracted a great deal of attention in the chemical reaction dynamics community over the past decade and still demonstrates unexpected results. Using velocity-map imaging of state-selected H2 products of H2CO photodissociation, Quinn et al. discovered the bimodal structure of rotational distribution of the other product fragment, CO. Quasiclassical trajectories showed that this bimodality originates from two distinctive reaction pathways that proceed by the trans or cis configuration of O–C–H⋯H, leading to high or low rotational excitations of CO, respectively. Whether such a mechanism is present in the many other chemical reactions for which roaming reaction pathways have been reported is yet to be determined. Science , this issue p. [1592][1] [1]: /lookup/doi/10.1126/science.abc4088

Looking at the bigger picture: Identifying the photoproducts of pyruvic acid at 193 nm.

Photodissociation of pyruvic acid (PA) was studied in the gas-phase at 193 nm using two complementary techniques. The time-sliced velocity map imaging arrangement was used to determine kinetic energy release distributions of fragments and estimate dissociation timescales. The multiplexed photoionization mass spectrometer setup was used to identify and quantify photoproducts, including isomers and free radicals, by their mass-to-charge ratios, photoionization spectra, and kinetic time profiles. Using these two techniques, it is possible to observe the major dissociation products of PA photodissociation: CO2, CO, H, OH, HCO, CH2CO, CH3CO, and CH3. Acetaldehyde and vinyl alcohol are minor primary photoproducts at 193 nm, but products that are known to arise from their unimolecular dissociation, such as HCO, H2CO, and CH4, are identified and quantified. A multivariate analysis that takes into account the yields of the observed products and assumes a set of feasible primary dissociation reactions provides a reasonable description of the photoinitiated chemistry of PA despite the necessary simplifications caused by the complexity of the dissociation. These experiments offer the first comprehensive description of the dissociation pathways of PA initiated on the S3 excited state. Most of the observed products and yields are rationalized on the basis of three reaction mechanisms: (i) decarboxylation terminating in CO2 + other primary products (∼50%); (ii) Norrish type I dissociation typical of carbonyls (∼30%); and (iii) O-H and C-H bond fission reactions generating the H atom (∼10%). The analysis shows that most of the dissociation reactions create more than two products. This observation is not surprising considering the high excitation energy (∼51 800 cm-1) and fairly low energy required for dissociation of PA. We find that two-body fragmentation processes yielding CO2 are minor, and the expected, unstable primary co-fragment, methylhydroxycarbene, is not observed because it probably undergoes fast secondary dissociation and/or isomerization. Norrish type I dissociation pathways generate OH and only small yields of CH3CO and HOCO, which have low dissociation energies and further decompose via three-body fragmentation processes. Experiments with d1-PA (CH3COCOOD) support the interpretations. The dissociation on S3 is fast, as indicated by the products' recoil angular anisotropy, but the roles of internal conversion and intersystem crossing to lower states are yet to be determined.

Laser-Induced Dissociation of the Monolayer of Adsorbed Methanol Molecules

Astronomical observations indicate a high abundance of methanol molecules in the gas phase of molecular-cloud dense cores, which cannot be explained by gas-phase chemical reactions only. A significant contribution to the methanol abundance should be provided by chemical reactions on the dust particle surface with subsequent desorption of the produced molecules into the gas phase. For the development and refinement of models involving these processes, laboratory studies of photo-induced processes occurring in the adsorbed material are necessary. In this paper, the experiment results of adsorbed methanol molecules are presented. A methanol molecule monolayer, physically adsorbed on fused silica surface cooled by liquid nitrogen (Т ∼ 100 K), was irradiated in high vacuum by nanosecond pulses of an excimer KrF laser with a fixed wavelength λ = 248 nm. The photodissociation products of three-photon laser excitation were recorded by a quadrupole mass spectrometer. Relative yields of photofragments H, OH, and CH3 were determined. Photolysis of partially deuterated CH3OH molecules has shown that hydrogen atoms can be ejected both from hydroxyl and methyl groups. In contrast to the isolated molecule photolysis in the gas phase and dissociation of the multilayer molecular coatings, photoexcitation of adsorbed methanol monolayer even in the energy region of 10 eV does not cause noticeable chemical transformations and does not lead to the formation of molecular components H2 and CH4. Due to existing astrochemical modeling problems, possible application methods of the obtained laboratory results are considered.

Spectroscopy and Two-Photon Dissociation of Jet-Cooled Pyruvic Acid.

The UV photodissociation of pyruvic acid (PA) is studied in molecular beams using time-of-flight (TOF) mass spectroscopy and time-sliced velocity map imaging (VMI) following excitation to the first absorption band (S1 ← S0) at 330-380 nm. CH3CO, HOCO, CO, CH3, and H are detected as photodissociation products. The photofragment yield (PFY) spectrum of the H product is recorded at 350-380 nm in He and Ar carrier gases. The spectrum shows sharp vibrational features reflecting the significant rotational cooling achieved in the molecular beam. It matches well the broad features observed in the room temperature absorption spectrum and indicates that the S1 state lives longer than a picosecond. The origin band of the S1 ← S0 transition is identified at 26 710 cm-1, and progressions in the CH3 and C-C torsional modes are tentatively assigned. Kinetic energy release (KER) and angular distributions of CH3CO, HOCO, CO, CH3, and H fragments indicate that additional photon absorption from S1 to the S2/S3 states is facile and is followed by rapid dissociation to the observed fragments. On the basis of the energetics of the different dissociation pathways and analyses of the observed KER distributions, three-body fragmentation processes are proposed as major contributors to the formation of the observed products.

Photodissociation of CO in the outflow of evolved stars

Context. Ultraviolet (UV) photodissociation of carbon monoxide (CO) controls the abundances and distribution of CO and its photodissociation products. This significantly influences the gas-phase chemistry in the circumstellar material around evolved stars. A better understanding of CO photodissociation in outflows also provides a more precise estimate of mass-loss rates. Aims. We aim to update the CO photodissociation rate in an expanding spherical envelope assuming that the interstellar radiation field (ISRF) photons penetrate through the envelope. This will allow us to precisely estimate the CO abundance distributions in circumstellar envelope around evolved stars. Methods. We used the most recent CO spectroscopic data to precisely calculate the depth dependency of the photodissociation rate of each CO dissociating line. We calculated the CO self- and mutual-shielding functions in an expanding envelope. We investigated the dependence of the CO profile on the five fundamental parameters mass-loss rate, the expansion velocity, the CO initial abundance, the CO excitation temperature, and the strength of the ISRF. Results. Our derived CO envelope size is smaller than the commonly used radius derived by Mamon et al. (1988, ApJ, 328, 797). The difference between results varies from 1 to 39% and depends on the H2 and CO densities of the envelope. We list two fitting parameters for a large grid of models to estimate the CO abundance distribution. We demonstrate that the CO envelope size can differ between outflows with the same effective content of CO, but different CO abundance, mass-loss rate, and the expansion velocity as a consequence of differing amounts of shielding by H2 and CO. Conclusions. Our study is based on a large grid of models employing an updated treatment of the CO photodissociation, and in it we find that the abundance of CO close to the star and the outflow density both can have a significant effect on the size of the molecular envelope. We also demonstrate that modest variations in the ISRF can cause measurable differences in the envelope extent.

The impact of bath gas composition on the calibration of photoacoustic spectrometers with ozone at discrete visible wavelengths spanning the Chappuis band

Abstract. Photoacoustic spectroscopy is a sensitive in situ technique for measuring the absorption coefficient for gas and aerosol samples. Photoacoustic spectrometer (PAS) instruments require accurate calibration by comparing the measured photoacoustic response with a known level of absorption for a calibrant. Ozone is a common calibrant of PAS instruments, yet recent work by Bluvshtein et al. (2017) has cast uncertainty on the validity of ozone as a calibrant at a wavelength of 405 nm. Moreover, Fischer and Smith (2018) demonstrate that a low O2 mass fraction in the bath gas can bias the measured PAS calibration coefficient to lower values for wavelengths in the range 532–780 nm. In this contribution, we present PAS sensitivity measurements at wavelengths of 405, 514 and 658 nm using ozone-based calibrations with variation in the relative concentrations of O2 and N2 bath gases. We find excellent agreement with the results of Fischer and Smith at the 658 nm wavelength. However, the PAS sensitivity decreases significantly as the bath gas composition tends to pure oxygen for wavelengths of 405 and 514 nm, which cannot be rationalised using arguments presented in previous studies. To address this, we develop a model to describe the variation in PAS sensitivity with both wavelength and bath gas composition that considers Chappuis band photodynamics and recognises that the photoexcitation of O3 leads rapidly to the photodissociation products O(3P) and O2(X, v > 0). We show that the rates of two processes are required to model the PAS sensitivity correctly. The first process involves the formation of vibrationally excited O3(X̃) through the reaction of the nascent O(3P) with bath gas O2. The second process involves the quenching of vibrational energy from the nascent O2(X, v > 0) to translational modes of the bath gas. Both of these processes proceed at different rates in collisions with N2 or O2 bath gas species. Importantly, we show that the PAS sensitivity is optimised for our PAS instruments when the ozone-based calibration is performed in a bath gas with a similar composition to ambient air and conclude that our methods for measuring aerosol absorption using an ozone-calibrated PAS are accurate and without detectable bias. We emphasise that the dependence of PAS sensitivity on bath gas composition is wavelength-dependent, and we recommend strongly that researchers characterise the optimal bath gas composition for their particular instrument.

Dynamical interference in the vibronic bond breaking reaction of HCO.

First-principles treatments of quantum molecular reaction dynamics have reached the level of quantitative accuracy even in cases with strong non-Born-Oppenheimer effects. This achievement permits the interpretation of puzzling experimental phenomena related to dynamics governed by multiple coupled potential energy surfaces. We present a combined experimental and theoretical study of the photodissociation of formyl radical (HCO). Oscillations observed in the distribution of product states are found to arise from the interference of matter waves-a manifestation analogous to Young's double-slit experiment.

Empirical assignment of absorbing electronic state contributions to OCS photodissociation product state populations from 214 to 248 nm

The photodissociation dynamics of OCS near 214 nm for the S(1D2) channel were studied using velocity map ion imaging. Earlier studies at this wavelength were extended with new measurements of vector correlations and reanalysis of the rotational distributions in the υ = 0 and υ = 1 CO products. The results were employed to identify the contributions of the 21A′ (A) and 11A″ (B) electronic state absorptions in OCS to the population of each CO final state. The contributions of the 11A″ (B) state to the CO (υ = 1) population was found to be ∼2.5 times smaller (fB=0.14±0.04) than previous theoretical estimates, and that to the CO (υ = 0) distribution ∼1.7 times smaller (fB=0.20±0.04). A similar analysis was applied to experimental rotational distributions from the literature at photolysis wavelengths of 223 nm, 230 nm, and 248 nm.

Photodissociation of Sodium Iodide Clusters Doped with Small Hydrocarbons.

Marine aerosols consist of a variety of compounds and play an important role in many atmospheric processes. In the present study, sodium iodide clusters with their simple isotope pattern serve as model systems for laboratory studies to investigate the role of iodide in the photochemical processing of sea‐salt aerosols. Salt clusters doped with camphor, formate and pyruvate are studied in a Fourier transform ion cyclotron resonance mass spectrometer (FT‐ICR MS) coupled to a tunable laser system in both UV and IR range. The analysis is supported by ab initio calculations of absorption spectra and energetics of dissociative channels. We provide quantitative analysis of IRMPD measurements by reconstructing one‐photon spectra and comparing them with the calculated ones. While neutral camphor is adsorbed on the cluster surface, the formate and pyruvate ions replace an iodide ion. The photodissociation spectra revealed several wavelength‐specific fragmentation pathways, including the carbon dioxide radical anion formed by photolysis of pyruvate. Camphor and pyruvate doped clusters absorb in the spectral region above 290 nm, which is relevant for tropospheric photochemistry, leading to internal conversion followed by intramolecular vibrational redistribution, which leads to decomposition of the cluster. Potential photodissociation products of pyruvate in the actinic region may be formed with a cross section of <2×10−20 cm2, determined by the experimental noise level.

Entanglement of the molecular photodissociation products at avoided crossings and conical intersections

This is a quantum mechanical study of the entanglement between pyrrolyl radical and the H-atom formed in the photodissociation of pyrrole. The parent molecule is photoexcited into the low lying 1πσ∗ states interacting with the ground electronic state via avoided crossings or conical intersections. The entanglement, studied in the time independent and the time dependent frameworks, is quantified using the von Neumann entropy which can be expressed in terms of the normalized internal state distributions of the pyrrolyl product. It is demonstrated that the von Neumann entropy is sensitive to the coupling between two electronic states and is substantially affected by Fano resonances in the dissociating molecule. The entropy generated at a conical intersection is generally larger than at an avoided crossing, although the main qualitative features are similar in both cases.

Carbon and oxygen isotopic fractionation in the products of low-temperature VUV photodissociation of carbon monoxide

The carbon and oxygen isotope fractionations occurring during photodissociation of carbon monoxide (CO) by vacuum ultraviolet photons (91–107 nm) at 80 K were measured and the isotopic fractionations due to direct and indirect predissociation processes individually quantified. The isotopic fractionations depend on the photodissociation wavelength. Slope values (δ'17O/δ'18O) in oxygen three-isotope space range from 0.75 to 1.1. The isotopic composition of the products depends on the dissociation dynamics at the upper electronic state (perturbation and coupling associated with that state), which in turn modulates the isotope effect (in this case an enrichment of minor isotopes) inside the gas column due to the saturation of major isotopologue (isotope self-shielding). An explanation in terms of isotope self-shielding would require a quantum yield of one for photodissociation of all isotopologues which is not consistent with the data.

Detection of CI line emission towards the oxygen-rich AGB star omi Ceti

We present the detection of neutral atomic carbon CI(3P1–3P0) line emission towards omi Cet. This is the first time that CI is detected in the envelope around an oxygen-rich M-type asymptotic giant branch (AGB) star. We also confirm the previously tentative CI detection around V Hya, a carbon-rich AGB star. As one of the main photodissociation products of parent species in the circumstellar envelope (CSE) around evolved stars, CI can be used to trace sources of ultraviolet (UV) radiation in CSEs. The observed flux density towards omi Cet can be reproduced by a shell with a peak atomic fractional abundance of 2.4 × 10−5 predicted based on a simple chemical model where CO is dissociated by the interstellar radiation field. However, the CI emission is shifted by ~4 km s−1 from the stellar velocity. Based on this velocity shift, we suggest that the detected CI emission towards omi Cet potentially arises from a compact region near its hot binary companion. The velocity shift could, therefore, be the result of the orbital velocity of the binary companion around omi Cet. In this case, the CI column density is estimated to be 1.1 × 1019 cm−2. This would imply that strong UV radiation from the companion and/or accretion of matter between two stars is most likely the origin of the CI enhancement. However, this hypothesis can be confirmed by high-angular resolution observations.

Cl‐Radical Formation in UV Photodissociation of Tetrachlorocyclopropene: A REMPI Study

AbstractCl atoms in both ground [Cl(2P3/2)] and excited spin orbit states [Cl*(2P1/2)] were detected as photodissociation products of tetrachlorocyclopropene (TCCP) at 235 nm by Resonance Enhanced Multiphoton Ionisation – Time of Flight (REMPI‐TOF) technique. From the experimental REMPI‐TOF profile, the anisotropy parameter (β) for both the channels was found to be nearly zero. TOF profiles of both the Cl and Cl* channels could be fitted into single component translational energy distributions, with average energy of 7.7 and 8.6 kcal mol−1 respectively. Time Dependent – Density Functional Theory (TD‐DFT) calculations using mpw1pw91/aug‐cc‐pVTZ level of theory suggested that, near 235 nm, the initially excited nCl—π* state of TCCP may cross over to the nearby dissociative states, from where Cl/Cl* can be formed directly. Otherwise, the excited molecules may relax rapidly to the ground state, where the lowest energy dissociation channel is the C—C bond cleavage (ΔH=30.4 kcal mol−1), as C—Cl bond breaking channels need high energy (ΔH=73.7 kcal mol−1 and 85.5 kcal mol−1). The biradical product of C—C bond breaking may rearrange to tetrachloroallene molecule with four equivalent C—Cl bonds, which can subsequently undergo C—Cl bond dissociation to produce Cl/Cl* atoms.

The Abundance of SiC2 in Carbon Star Envelopes: Evidence that SiC2 is a gas-phase precursor of SiC dust.

Silicon carbide dust is ubiquitous in circumstellar envelopes around C-rich AGB stars. However, the main gas-phase precursors leading to the formation of SiC dust have not yet been identified. The most obvious candidates among the molecules containing an Si-C bond detected in C-rich AGB stars are SiC2, SiC, and Si2C. To date, the ring molecule SiC2 has been observed in a handful of evolved stars, while SiC and Si2C have only been detected in the C-star envelope IRC +10216. We aim to study how widespread and abundant SiC2, SiC, and Si2C are in envelopes around C-rich AGB stars and whether or not these species play an active role as gas-phase precursors of silicon carbide dust in the ejecta of carbon stars. We carried out sensitive observations with the IRAM 30m telescope of a sample of 25 C-rich AGB stars to search for emission lines of SiC2, SiC, and Si2C in the λ 2 mm band. We performed non-LTE excitation and radiative transfer calculations based on the LVG method to model the observed lines of SiC2 and to derive SiC2 fractional abundances in the observed envelopes. We detect SiC2 in most of the sources, SiC in about half of them, and do not detect Si2C in any source, at the exception of IRC +10216. Most of these detections are reported for the first time in this work. We find a positive correlation between the SiC and SiC2 line emission, which suggests that both species are chemically linked, the SiC radical probably being the photodissociation product of SiC2 in the external layer of the envelope. We find a clear trend in which the denser the envelope, the less abundant SiC2 is. The observed trend is interpreted as an evidence of efficient incorporation of SiC2 onto dust grains, a process which is favored at high densities owing to the higher rate at which collisions between particles take place. The observed behavior of a decline in the SiC2 abundance with increasing density strongly suggests that SiC2 is an important gas-phase precursor of SiC dust in envelopes around carbon stars.

ALMA Detection of Interstellar Methoxymethanol (CH3OCH2OH)

Abstract We report the detection of interstellar methoxymethanol (CH3OCH2OH) in Atacama Large Millimeter/submillimeter Array (ALMA) Bands 6 and 7 toward the MM1 core in the high-mass star-forming region NGC 6334I at ∼0.″1–1″ spatial resolution. A column density of 4(2) × 1018 cm−2 at T ex = 200 K is derived toward MM1, ∼34 times less abundant than methanol (CH3OH), and significantly higher than predicted by astrochemical models. Probable formation and destruction pathways are discussed, primarily through the reaction of the CH3OH photodissociation products, the methoxy (CH3O) and hydroxymethyl (CH2OH) radicals. Finally, we comment on the implications of these mechanisms on gas-phase versus grain-surface routes operative in the region, and the possibility of electron-induced dissociation of CH3OH rather than photodissociation.

A measurement of the photoionization cross section of CH2Cl via photofragment translational spectroscopy of dichloromethane

We characterize the 157nm photodissociation of dichloromethane (CH2Cl2) using photofragment translational spectroscopy, detecting the products with photoionization mass spectrometry. The major photodissociation products are Cl+CH2Cl and HCl+CHCl. Comparing the integrated ion signal at m/e=35 (Cl+) and m/e=49 (CH2Cl+) gives the partial photoionization cross section of CH2Cl to parent ion to be σ=26.6Mb at 13.7eV. This value is a quantitative anchor point of the energy-dependent photoionization cross section. In recent work we used this result to quantify the product branching to HCO+CH2Cl in the photodissociation of 2-chloroacetaldehyde.

Diffusion of Formaldehyde on Rutile TiO2(110) Assisted by Surface Hydroxyl Groups

As the photo-dissociation product of methanol on the TiO2 (110) surface, the diffusion and desorption processes of formaldehyde (HCHO) were investigated by using scanning tunneling microscope (STM) and density functional theory (DFT). The molecular-level images revealed the HCHO molecules could diffuse and desorb on the surface at 80 K under UV laser irradiation. The diffusion was found to be mediated by hydrogen adatoms nearby, which were produced from photodissociation of methanol. Diffusion of HCHO was significantly decreased when there was only one H adatom near the HCHO molecule. Furthermore, single HCHO molecule adsorbed on the bare TiO2(110) surface was quite stable, little photo-desorption was observed during laser irradiation. The mechanism of hydroxyl groups assisted diffusion of formaldehyde was also investigated using theoretical calculations.

Highly Nuclear-Spin-Polarized Deuterium Atoms from the UV Photodissociation of Deuterium Iodide

We report a novel highly spin-polarized deuterium (SPD) source, via the photodissociation of deuterium iodide at 270nm. I(^{2}P_{3/2}) photofragments are ionized with m-state selectivity, and their velocity distribution measured via velocity-map slice imaging, from which the D polarization is determined. The process produces ∼100% electronically polarized D at the time of dissociation, which is then converted to ∼60% nuclear D polarization after ∼1.6 ns. These production times for SPD allow collision-limited densities of ∼10^{18} cm^{-3} and at production rates of ∼10^{21} s^{-1} which are 10^{6} and 10^{4} times higher than conventional (Stern-Gerlach separation) methods, respectively. We discuss the production of SPD beams, and combining high-density SPD with laser fusion, to investigate polarized D-T, D-^{3}He, and D-D fusion.

Photochemical synthesis and formation mechanisms of polyvinyl acetate–CoIII(salen) “dormant” chains during “living” radical polymerization

The formation of “dormant” chains due to the acceptance of the photodissociation products of vinyl acetate by stable CoII(salen) radicals was studied. The rate of formation of “dormant” chains was found experimentally to be considerably higher than that of initiating radicals upon the initiator-free photopolymerization of vinyl acetate. A high rate of formation of “dormant” chains is caused by supression of the cell effect in the presence of a strong inhibitor. A kinetic scheme was proposed which allowed drawing up a mechanism of formation of “dormant” chains as a mathematical model suitable for the quantitative description of the process. The comparison between the absorption spectra of the reaction system at different reaction steps and the quantum chemically calculated spectra of forming “dormant” chains showed the polymer chain initiation to afford the acyl radical. The constants for the growing radicals–CoII(salen) interaction, the quantum yield of the Co–C bond dissociation in “dormant” chains, and the quantum yield of the initiator-free vinyl acetate photopolymerization were determined.

Tracking the photodissociation dynamics of liquid nitromethane at 266 nm by femtosecond time-resolved broadband transient grating spectroscopy

Femtosecond time-resolved transient grating (TG) technique was employed to get insight into the photodissociation mechanism of liquid nitromethane (NM). Broadband white-light continuum was introduced as the probe to observe the evolution of electronic excited states of NM molecules and the formation of photodissociation products simultaneously. The reaction channel of liquid NM under 266nm excitation was obtained that NM molecules in excited state S2 relax through two channels: about 73% relax to low lying S1 state through S2/S1 internal conversion with a time constant of 0.24ps and then go back to the ground state through S1/S0 internal conversion; the other 27% will dissociate with a time constant of 2.56ps. NO2 was found to be one of the products from the experimental TG spectra, which confirmed that C–N bond rupture was the primary dissociation channel of liquid NM.