Photodissociation Processes Research Articles

Local changes in arterial oxygen saturation induced by visible and near-infrared light radiation

In this study, we investigate the efficiency of laser radiation on oxyhemoglobin (HbO2) rate in blood vessels and its wavelength dependence. The results of in vivo experimental measurements of the laser-induced photodissociation of HbO2 in cutaneous blood vessels in the visible and near-infrared (IR) spectral range are presented. Arterial oxygen saturation (SpO2) was measured by a method of fingertip pulse oximetry, which is based on the measurement of the modulated pulse wave of the blood. The light irradiating the finger was provided by corresponding light-emitting diodes (LED) at 15 wavelengths in the 400-940nm spectrum range. Statistical results with a value of p < 0.05 were viewed as being significant for all volunteers. The results show that there is a decrease in SpO2 in the blood under the influence of the transcutaneous laser irradiation. Three maxima in the spectral range (530, 600, and 850nm) are revealed, wherein decrease in the relative concentration of SpO2 reaches 5% ± 0.5%. Near-IR radiation plays a dominant role in absorption of laser radiation by oxyhemoglobin in deeper layers of tissue blood vessels. The obtained data correlate with the processes of light propagation in biological tissue. The observed reduction in SpO2 indicates the process of photodissociation of HbO2 in vivo and may result in local increase in O2 in the tissue. Such laser-induced enrichment of tissue oxygenation can be used in phototherapy of pathologies, where the elimination of local tissue hypoxia is critical.

Ab initio implementation of quantum trajectory mean-field approach and dynamical simulation of the N2CO photodissociation.

In this work, the recently introduced quantum trajectory mean-field (QTMF) approach is implemented and employed to explore photodissociation dynamics of diazirinone (N2CO), which are based on the high-level ab initio calculation. For comparison, the photodissociation process has been simulated as well with the fewest-switches surface hopping (FSSH) and the ab initio multiple spawning (AIMS) methods. Overall, the dynamical behavior predicted by the three methods is consistent. The N2CO photodissociation at λ > 335 nm is an ultrafast process and the two C-N bonds are broken in a stepwise way, giving birth to CO and N2 as the final products in the ground state. Meanwhile, some noticeable differences were found in the QTMF, FSSH, and AIMS simulated time constants for fission of the C-N bonds, excited-state lifetime, and nonadiabatic transition ratios in different intersection regions. These have been discussed in detail. The present study provides a clear evidence that direct ab initio QTMF approach is one of the reliable tools for simulating nonadiabatic dynamics processes.

Quantum Interference Effects Theoretically Found in the Photodissociation Processes of the Second Absorption Bands of ICl and IBr Molecules.

Some quantum interference effects are exposed directly in experiments, but others are not and remain just hidden and thus require thorough theoretical analysis to be exposed. In this respect, the second absorption bands of IX (X = Cl, Br) molecules show an interesting behavior in the photofragment anisotropy of the lowest I((2)P3/2)+X((2)P3/2) product channel; it changes from strongly parallel distribution on the shorter wavelength side to strongly perpendicular distribution on the longer wavelength side. Because the responsible perpendicular third Ω = 1 (1(III)) excited state correlating adiabatically to this product channel has only a weak absorption, the parallel component flux yielding the same products must be comparatively weak, even though the responsible parallel excitations to the 0(+)(III) and/or 0(+)(IV) excited states have strong absorptions. In the present theoretical study, the branching ratios and the anisotropy parameters have been obtained using the spin-orbit configuration interaction method combined with the quantum mechanical wavepacket and semiclassical approaches. Significant quantum interference effect between the two dissociative de Broglie waves on the 0(+)(III) and 0(+)(IV) potential curves has been found through the avoided crossing, and the weak parallel flux to the ground-state product channel has been explained by their destructive interference character. This interference effect has weak excitation energy dependence due to rather unique behavior of their almost parallel potential curves from the Franck-Condon to avoided crossing regions.

Potential energy curves of the ground and excited states of molecular hydrogen calculated by quasicomplete bases

SynopsisThe “quasi-complete” basis calculation scenario for atomic excited states were extended to molecular system. We successfully construct the “quasi-complete” bases which consist of the spectroscopy orbitals determined by the key physical states and the pseudo-orbitals determined by the import electron correlations for the hydrogen molecule of H2. By using such “quasi-complete” bases, the potential energy curves of the ground and highly excited states of the H2 has been calculated and obtained in large scale 0 ≤ RH-H ≤ 120 Bohr. This will be very useful for further studies on spectroscopy, photoionization and photo-dissociation processes.

Triggering photochemical processes in frozen extraterrestrial worlds by soft X-rays

SynopsisWe presents and experimental investigation on the effects produced by broad band soft X-rays (combined with a small fraction of vacuum ultra violet photons and possibly secondary elec-trons) on the surface of three moons of giant planets: Europa, Titan and Enceladus. Such environ-ments are constantly exposed to space ionizing agents (UV and soft X-rays photons, electrons and ions) allowing photodissociation processes, surface photochemistry and prebiotic chemistry. The processing of such spatial ices have promoted an enhancement in the chemical complexity, similar what may have happened in the early earth triggering the arising of life.

Methyl formate ice stimulated by X-rays in protoplanetary discs

SynopsisWe study experimentally the processes of photoionization, photodissociation and photodesorption in methyl formate ice due to interaction of soft X-rays at two energy ranges, from 284 to 298 eV and from 528 to 546 eV, around the C ls→π* and O ls→π* resonances. The photodesorption ion yield was obtained for some of the desorbed species of the ice surface, like CH+, HCO+ and H3CO+. The extrapolation of these results to an astrophysical environment, as the protoplanetary disc of TW Hydrae star, is presented

Multiple-photon absorption in cooled molecular beams of acenes compounds at 266 and 355 nm laser radiation

SynopsisThe cooled molecular beams of acenes compounds as naphthalene C10H8, anthracene C14H10, and ben- zantracene C18H12, were produced by adiabatic expansion in high vacuum chamber, 10-8 torr. Molecular beams interacted with laser radiation of 266 and 355 nm, from a Ng:YAG laser at intensities up to 1010 W·cm-2 At higher intensities the multiple photon absorption was possible. The number of photons was calculated using Keldysh approach. The dominant photophysics of photon-molecule interactions were photodissociation and photoionization processes depending on the wavelength used during the experiment.

Direct Signature of Light-Induced Conical Intersections

SynopsisBy analyzing the photodissociation process of the D+2 molecule, we found a robust effect in the angular distribution of the photofragments which serves as a direct signature of the light-induced conical intersection (LICI) providing undoubted evidence for its existence.

VUV Photodissociation Dynamics of Nitrous Oxide: The O((1)SJ=0) and O((3)PJ=2,1,0) Product Channels.

Vacuum ultraviolet photodissociation dynamics of nitrous oxide was investigated using the time-sliced velocity ion imaging technique. Images of the O((1)SJ=0) and the O((3)PJ=2,1,0) products were measured at nine photolysis wavelengths from 124.44 to 133.20 nm, respectively. Three main dissociation channels: O((1)S0) + N2(X(1)Σg(+)), O((3)PJ=2,1,0) + N2(A(3)Σu(+)), and O((3)PJ=2,1,0) + N2(B(3)Πg) were observed and identified in the product images where vibrational states of N2 were fully resolved. Product total kinetic energy releases and angular distributions were acquired. In all product channels, the branching ratios of vibrational states of N2 products were determined. In addition, the O((3)PJ=2,1,0) + N2(A(3)Σu(+))/O((3)PJ=2,1,0) + N2(B(3)Πg) branching ratios were determined. We found that in the O((3)PJ=2,1,0) channels the O((3)PJ=2,1,0) + N2(B(3)Πg) channel becomes dominant at long photolysis wavelength, indicating a strong coupling between the singlet D((1)Σg(+)) state and the triplet (3)Π state. For both O((1)S0) and O((3)PJ=2,1,0) products, the derived angular anisotropy parameters (β values) are very close to 2 at lower vibrational states of the correlated N2 electronic states and gradually decrease with the increasing vibrational quantum number. These behaviors suggest that the photodissociation processes are primarily governed by a fast dissociation in a linear geometry, while the N2 products at excited vibrational states are very likely produced via a more bent transition state.

Photodissociation of D2 (+) induced by linearly chirped laser pulses.

Recently, it has been revealed that so-called light-induced conical intersections (LICIs) can be formed both by standing or by running laser waves even in diatomic molecules. Due to the strong nonadiabatic couplings, the existence of such LICIs has significant impact on the dynamical properties of a molecular system. In our former studies, the photodissociation process of the D2 (+) molecule was studied initiating the nuclear dynamics both from individual vibrational levels and from the superposition of all the vibrational states produced by ionizing D2. In the present work, linearly chirped laser pulses were used for initiating the dissociation dynamics of D2 (+). In contrast to the constant frequency (transform limited) laser fields, the chirped pulses give rise to LICIs with a varying position according to the temporal frequency change. To demonstrate the impact of these LICIs on the dynamical properties of diatomics, the kinetic energy release spectra, the total dissociation probabilities, and the angular distributions of the D2 (+) photofragments were calculated and discussed.

ChemInform Abstract: Strong Field Laser Control of Photochemistry

AbstractReview: 141 refs.

Photodissociation at various wavelengths: fragmentation studies of oxazine 170 using nanosecond laser pulses.

The fragmentation of oxazine 170, a rhodamine-type dye, has been investigated by means of collisions and photodissociation with visible and ultraviolet radiation in a Fourier transform ion cyclotron resonance mass spectrometer. Because of an improved experimental setup, the photodissociation processes of stored ions are measured with high intensity with respect to the absorbed photons. By isotope labelling and quantum chemical calculations, the various fragmentation mechanisms are investigated. It is shown that the most important intermediate ion structure leading to the various ionic products is an even-electron azarine cation. Several new fragmentation mechanisms have been unveiled for the first time.

Study of acoustic fingerprinting of nitromethane and some triazole derivatives using UV 266 nm pulsed photoacoustic pyrolysis technique

We report a comparative study of acoustic fingerprints of nitromethane, nitrobenzene and some nitro rich triazole derivatives using pulsed photoacoustic technique. UV 266nm wavelength i.e. Fourth harmonic of Q-switched Nd: YAG laser having pulse duration 7ns and 10Hz repetition rate is employed to record the time resolved PA spectrum. The PA fingerprint is produced due to absorption of incident UV light by molecule itself and photo dissociation of nitromethane and nitrobenzene at room temperature while in case of triazole it is attributed to the combination of thermal and photo-dissociation process. The entire dissociation process follows the root of cleavage of C-NO2 bond to produce free NO, NO2 and other by product gases due to π∗←n excitation. In addition, we have studied the thermal stability criteria of nitro rich triazoles based on the quality factor of acoustic resonance frequencies of the PA cavity. We have also studied the effect of data acquisition time to ascertain the decay behavior of HEMs samples.

Direct Signature of Light-Induced Conical Intersections in Diatomics.

Nonadiabatic effects are ubiquitous in physics, chemistry, and biology. They are strongly amplified by conical intersections (CIs), which are degeneracies between electronic states of triatomic or larger molecules. A few years ago it was revealed that CIs in molecular systems can be formed by laser light, even in diatomics. Because of the prevailing strong nonadiabatic couplings, the existence of such laser-induced conical intersections (LICIs) may considerably change the dynamical behavior of molecular systems. By analyzing the photodissociation process of the D2+ molecule carefully, we found a robust effect in the angular distribution of the photofragments that serves as a direct signature of the LICI, providing undoubted evidence of its existence.

Enhanced photochemistry of ethyl chloride on Ag nanoparticles.

Enhanced photodecomposition of ethyl chloride (EC) adsorbed on SiO2/Si (100) supported silver nanoparticles (Ag NPs) under ultrahigh vacuum (UHV) conditions has been studied in order to assess the potential contribution of plasmonic effects. The cross section for photodecomposition of EC and overall photoyield were found to increase with increasing photon energy regardless of the plasmon resonant wavelength and with Ag coverage without any noticeable particle size effect. The influence of EC-Ag NPs separation distance on the rate of EC decomposition was studied in order to examine potential local electric field influence on the photodissociation process. Long (∼5 nm) photoactivity decay distance has been observed which excludes local surface plasmon dominance in the photodecomposition event. These findings suggest that the alignment of excited electron energy and adsorbate affinity levels is central for efficient photochemical reactions, whereas short-range electric field enhancement by plasmon excitation on top and at the immediate vicinity of silver nanoparticles does not have any measurable effect.

Quantum and classical non-adiabatic dynamics of LiNe photodissociation

The 3D photodissociation dynamics of LiNe system is investigated by quantum calculations using the multi-configuration time-dependent Hartree (MCTDH) method and by classical simulations with the trajectory surface hopping (TSH) approach. Six electronic states of A’ symmetry and two states of A” symmetry are involved in the process. Couplings in the excitation region and two conical intersections in the vicinity of the Franck–Condon zone control the non-adiabatic nuclear dynamics. A diabatic representation including all the states and the couplings is determined. Diabatic and adiabatic populations calculated for initial excitation to pure diabatic and adiabatic states lead to a clear understanding of the mechanisms governing the non-adiabatic photodissociation process. The classical and quantum photodissociation cross-sections for absorption in two adiabatic states of the A’ symmetry are calculated. A remarkable agreement between quantum and classical results is obtained regarding the populations and the absorption cross-sections.

Photodissociation ofBr2molecules in an intense femtosecond laser field

We experimentally demonstrate the photodissociation process of ${\mathrm{Br}}_{2}$ molecules in the intense femtosecond laser field by a dc-sliced ion velocity map imaging technique. We show that four fragment ions $\mathrm{B}{\mathrm{r}}^{n+}\phantom{\rule{0.16em}{0ex}}\left(n=1--4\right)$ are observed, and their kinetic energy increases while their angular distribution decreases with the increase of the charge number. We prove that the low (or high) charged fragment ions result from the photodissociation of the low (or high) charged parent ions. We explain the changes of the kinetic energy and angular distribution in these fragment ions by considering the potential energy curves of these parent ions that involve both the interaction of the Coulomb repulsive energy and chemical bonding energy. We also explain the experimental observation that the measured kinetic energy release in the experiment is much smaller than the theoretical calculation by enhanced ionization at a critical distance.

Photodissociation of OCS: deviations between theory and experiment, and the importance of higher order correlation effects.

The photodissociation of carbonyl sulfide (OCS) was investigated theoretically in a series of studies by Schmidt and co-workers. Initial studies [J. A. Schmidt, M. S. Johnson, G. C. McBane, and R. Schinke, J. Chem. Phys. 136, 131101 (2012); J. A. Schmidt, M. S. Johnson, G. C. McBane, and R. Schinke, J. Chem. Phys. 137, 054313 (2012)] found photodissociation in the first UV-band to occur mainly by excitation of the 2(1)A' (A) excited state. However, in a later study [G. C. McBane, J. A. Schmidt, M. S. Johnson, and R. Schinke, J. Chem. Phys. 138, 094314 (2013)] it was found that a significant fraction of photodissociation must occur by excitation of 1(1)A″ (B) excited state to explain the product angular distribution. The branching between excitation of the A and B excited states is determined by the magnitude of the transition dipole moment vectors in the Franck-Condon region. This study examines the sensitivity of these quantities to changes in the employed electronic structure methodology. This study benchmarks the methodology employed in previous studies against highly correlated electronic structure methods (CC3 and MRAQCC) and provide evidence in support of the picture of the OCS photodissociation process presented in [G. C. McBane, J. A. Schmidt, M. S. Johnson, and R. Schinke, J. Chem. Phys. 138, 094314 (2013)] showing that excitation of A and B electronic states both contribute significantly to the first UV absorption band of OCS. In addition, this study presents evidence in support of the assertion that the A state potential energy surface employed in previous studies underestimates the energy at highly bent geometries (γ ∼ 70°) leading to overestimated rotational energy in the product CO.

Photodissociation dynamics of phenol: multistate trajectory simulations including tunneling.

We report multistate trajectory simulations, including coherence, decoherence, and multidimensional tunneling, of phenol photodissociation dynamics. The calculations are based on full-dimensional anchor-points reactive potential surfaces and state couplings fit to electronic structure calculations including dynamical correlation with an augmented correlation-consistent polarized valence double-ζ basis set. The calculations successfully reproduce the experimentally observed bimodal character of the total kinetic energy release spectra and confirm the interpretation of the most recent experiments that the photodissociation process is dominated by tunneling. Analysis of the trajectories uncovers an unexpected dissociation pathway for one quantum excitation of the O-H stretching mode of the S1 state, namely, tunneling in a coherent mixture of states starting in a smaller ROH (∼0.9-1.0 Å) region than has previously been invoked. The simulations also show that most trajectories do not pass close to the S1-S2 conical intersection (they have a minimum gap greater than 0.6 eV), they provide statistics on the out-of-plane angles at the locations of the minimum energy adiabatic gap, and they reveal information about which vibrational modes are most highly activated in the products.

Visible spectrum photofragmentation of O₃⁻(H₂O)n, n ≤ 16.

Photofragmentation of ozonide solvated in water clusters, O3(-)(H2O)n, n ≤ 16, has been studied as a function of photon energy as well as the degree of solvation. Using mass selection, the effect of the presence of the solvent molecule on the O3(-) photodissociation process is assessed one solvent molecule at a time. The O3(-) acts as a visible light chromophore within the water cluster, namely the O3(-)(H2O) total photodissociation cross-section exhibits generally the same photon energy dependence as isolated O3(-) throughout the visible wavelength range studied (430-620 nm). With the addition of a single solvent molecule, new photodissociation pathways are opened, including the production of recombined O3(-). As the degree of solvation of the parent anion increases, recombination to O3(-)-based products accounts for close to 40% of photoproducts by n = 16. The remainder of the photoproducts exist as O(-)-based; no O2(-)-based products are observed. Upper bounds on the O3(-) solvation energy (530 meV) and the O(-)-OO bond dissociation energy in the cluster (1.06 eV) are derived.