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The site-specific hydrogen-atom elimination mechanism previously reported for photoexcited ethyl radicals (CH3CH2) [D. V. Chicharro et al., Chem. Sci., 2019, 10, 6494] is interrogated in the photodissociation of the ethyl isotopologues CD3CD2, CH3CD2 and CD3CH2 through the velocity map imaging (VMI) detection of the produced hydrogen- and deuterium-atoms. The radicals, generated in situ from photolysis of a precursor using the same laser pulse employed in their excitation to Rydberg states, decompose along the Ca-H/D and Cb-H/D reaction coordinates through coexisting statistical and site-specific mechanisms. The experiments are carried out at two excitation wavelengths, 201 and 193 nm. The comparison between both sets of results provides accurate information regarding the primary role in the site-specific mechanism of the radical internal reservoir. Importantly, at 193 nm excitation, higher energy dissociation channels (not observed at 201 nm) producing low-recoil H/Datoms become accessible. High-level ab initio calculations of potential energy curves and the corresponding non-adiabatic interactions allow us to rationalize the experimental results in terms of competitive non-adiabatic decomposition paths. Finally, the adiabatic behavior of the conical intersections in the face of several vibrational modes – the so-called vibrational promoting modes – is discussed., Depto. de Química Física, Fac. de Ciencias Químicas, TRUE, pub, Descuento UCM

By coherent control, the yield of photodissociation reactions can be maximized, starting in a suitable superposition of vibrational states. In ultrafast processes, the interfering pathways are born from the early vibrational coherences in the ground electronic potential. We interpret their effect from a purely classical picture, in which the correlation between the initial position and momentum helps to synchronize the vibrational dynamics at the Franck¿Condon window when the pulse is at its maximum intensity. In the quantum domain, we show that this localization in time and space is mediated by dynamic squeezing of the wave packet.

Clocking of electronically and vibrationally state-resolved channels of the fast photodissociation of CH3I in the A-band is re-examined in a combined experimental and theoretical study. Experimentally, a femtosecond pump-probe scheme is employed in the modality of resonant probing by resonance enhanced multiphoton ionization (REMPI) of the methyl fragment in different vibrational states and detection through fragment velocity map ion (VMI) imaging as a function of the time delay. We revisit excitation to the center of the A-band at 268 nm and report new results for excitation to the blue of the band center at 243 nm. Theoretically, two approaches have been employed to shed light into the observations: first, a reduced dimensionality 4D nonadiabatic wavepacket calculation using the potential energy surfaces by Xie et al. [J. Phys. Chem. A 104, 1009 (2000)]; and second, a full dimension 9D trajectory surface-hopping calculation on the same potential energy surfaces, including the quantization of vibrational states of the methyl product. In addition, high level ab initio electronic structure calculations have been carried out to describe the CH3 3pz Rydberg state involved in the (2 + 1) REMPI probing process, as a function of the carbon-iodine (C–I) distance. A general qualitative agreement is obtained between experiment and theory, but the effect of methyl vibrational excitation in the umbrella mode on the clocking times is not well reproduced. The theoretical results reveal that no significant effect on the state-resolved appearance times is exerted by the nonadiabatic crossing through the conical intersection present in the first absorption band. The vibrationally state resolved clocking times observed experimentally can be rationalized when the (2 + 1) REMPI probing process is considered. None of the other probing methods applied thus far, i.e., multiphoton ionization photoelectron spectroscopy, soft X-ray inner-shell photoelectron spectroscopy, VUV single, Depto. de Química Física, Fac. de Ciencias Químicas, TRUE, pub

Photodissociation of the CH2I radical and the CH2I+ cation is studied by means of high-level ab initio calculations, including spin-orbit effects. Potential-energy curves (PEC) along the dissociating bond distances involved in some fragmentation pathways of these species are computed for the ground and several excited electronic states. Based on the PECs obtained, the possible photodissociation mechanisms are analyzed and suggested. Significant differences are found between the fragmentation dynamics of the neutral radical and that of the cation. While a relatively simple dissociation dynamics is predicted for CH2I, more complex fragmentation mechanisms involving internal conversion and couplings between different excited electronic states are expected for CH2I+. The species studied here are relevant to atmospheric chemistry, and the present work can help to understand better how their photodissociation may affect chemical processes in the atmosphere.

The site-specific hydrogen-atom elimination mechanism previously reported for photoexcited ethyl radicals (CH3CH2) [D. V. Chicharro et al., Chem. Sci., 2019, 10, 6494] is interrogated in the photodissociation of the ethyl isotopologues CD3CD2, CH3CD2 and CD3CH2 through the velocity map imaging (VMI) detection of the produced hydrogen- and deuterium-atoms. The radicals, generated in situ from photolysis of a precursor using the same laser pulse employed in their excitation to Rydberg states, decompose along the Ca-H/D and Cb-H/D reaction coordinates through coexisting statistical and site-specific mechanisms. The experiments are carried out at two excitation wavelengths, 201 and 193 nm. The comparison between both sets of results provides accurate information regarding the primary role in the site-specific mechanism of the radical internal reservoir. Importantly, at 193 nm excitation, higher energy dissociation channels (not observed at 201 nm) producing low-recoil H/Datoms become accessible. High-level ab initio calculations of potential energy curves and the corresponding non-adiabatic interactions allow us to rationalize the experimental results in terms of competitive non-adiabatic decomposition paths. Finally, the adiabatic behavior of the conical intersections in the face of several vibrational modes – the so-called vibrational promoting modes – is discussed., Depto. de Química Física, Fac. de Ciencias Químicas, TRUE, pub, Descuento UCM

The photodecomposition of methanimine (CH$_2$NH) in the interstellar medium through several possible pathways is investigated by means of high-level multireference configuration interaction {\it ab initio} calculations. Among them are photodissociation pathways involving hydrogen-atom elimination from both the CH$_2$ and NH groups, and fragmentation into CH$_2$ and NH. Potential-energy curves for the ground and several excited electronic states, as well as nonadiabatic couplings between them, are calculated. Possible dissociation mechanisms are discussed for the different pathways. It is found that the minimum excitation energy required for methanimine dissociation is above $7$ eV. By using a two-dimensional representation of methanimine, the CH$_2$NH $\rightarrow$ CHNH$_2$ isomerization is explored as an additional methanimine decomposition pathway. Hydrogen-atom elimination from the CH$_2$ group is also investigated along the isomerization pathway. The results show that the isomerization proceeds by overcoming a transition state that in the first two excited states would require excitation energies similar to or somewhat lower than the typical minimum energies needed for breaking the molecule through the fragmentation pathwys. Therefore, CH$_2$NH $\rightarrow$ CHNH$_2$ isomerization can effectively contribute to methanimine decomposition, competing efficiently with the photodissociation pathways. The radiation content present in the interstellar medium makes possible the occurrence of all the pathways studied.

Photodissociation of the vinyl radical through pathways CHCH → CHC + H, CHCH → CHCH + H, and CHCH → CH + CH is investigated by means of high-level ab initio calculations. Potential-energy curves (PECs) along the corresponding dissociating bond distance associated with the ground and several excited electronic states involved in the above fragmentation pathways, as well as the nonadiabatic couplings connecting the different states, are obtained. The findings of several experiments on vinyl photodissociation performed at different excitation wavelengths are analyzed and explained qualitatively in the light of the present PECs. A two-dimensional representation (consisting of radial and angular coordinates to represent one of the H atoms of the CH group) is also used to calculate the electronic states. The surfaces obtained reflect a rich variety of conical intersections, exit barriers, and nonadiabatic couplings leading to predissociation in different regions of energy and of the two coordinates, suggesting a complex photodissociation dynamics of the CHCH → CHCH + H pathway, with rather different fragmentation mechanisms involved. The two-dimensional results also provide interesting information on the mechanism of in-plane hydrogen migration from the CH group to the CH one through a high-lying transition state.

A coherent control scheme is suggested to modify the output of photodissociation in a polyatomic system. The performance of the scheme is illustrated by applying it to the ultrafast photodissociation of CH3I in the A-band. The control scheme uses a pump laser weak field that combines two pulses of a few femtoseconds delayed in time. By varying the time delay between the pulses, the shape of the laser field spectral profile is modulated, which causes a change in the initial relative populations excited by the pump laser to the different electronic states involved in the photodissociation. Such a change in the relative populations produces different photodissociation outputs, which is the basis of the control achieved. The degree of control obtained over different photodissociation observables, like the branching ratio between the two dissociation channels of CH3I yielding I(2P3/2) and I*(2P1/2) and the fragment angular distributions associated with each channel, is investigated. These magnitudes are found to oscillate strongly with the time delay, with the branching ratio changing by factors between two and three. Substantial variations of the angular distributions also indicate that the scheme provides a high degree of control. Experimental application of the scheme to general polyatomic photodissociation processes should be straightforward.

A unified weak-field control scheme to modify the two properties that determine the whole behavior of a resonance state, namely the lifetime and the asymptotic fragment distribution produced upon resonance decay, is proposed. Control is exerted through quantum interference induced between overlapping resonances of the system, by exciting two different energies at which the resonances overlap. The scheme applies a laser field consisting of a first pulse that excites the energy of the resonance to be controlled, and two additional pulses that excite another different energy to induce interference, with a delay time with respect to the first pulse. Each of the two additional pulses is used to control one of the two resonance properties, by adjusting its corresponding delay time: With a relatively short delay time the second pulse controls the resonance lifetime, while with a very long delay time the third pulse modifies the asymptotic fragment distribution produced. The efficiency of the control of each resonance property is found to be strongly dependent on the choice of the second interfering energy, which allows for a more flexible control optimization by choosing a different energy for each property. The theory underlaying the interference mechanism of the control scheme is developed and presented, and is applied to analyze and explain the results obtained. The present scheme thus appears as a useful tool of control of resonance-mediated molecular processes.

European Commission, Consejo Superior de Investigaciones Cientificas, Ministerio de Economía y Competitividad (España), Ministerio de Ciencia e Innovación (España), Depto. de Química Física, Fac. de Ciencias Químicas, TRUE, pub

CRUE-CSIC (Acuerdos Transformativos 2021), VUV photoionization of the CHnI radicals (with n = 0, 1, and 2) is investigated by means of synchrotron radiation coupled with a double imaging photoion-photoelectron coincidence spectrometer. Photoionization efficiencies and threshold photoelectron spectra (TPES) for photon energies ranging between 9.2 and 12.0 eV are reported. An adiabatic ionization energy (AIE) of 8.334 ± 0.005 eV is obtained for CH2I, which is in good agreement with previous results [8.333 ± 0.015 eV, Sztáray et al. J. Chem. Phys. 2017, 147, 013944], while for CI an AIE of 8.374 ± 0.005 eV is measured for the first time and a value of ∼8.8 eV is estimated for CHI. Ab initio calculations have been carried out for the ground state of the CH2I radical and for the ground state and excited states of the radical cation CH2I + , including potential energy curves along the C−I coordinate. Franck−Condon factors are calculated for transitions from the CH2I(X̃ 2 B1) ground state of the neutral radical to the ground state and excited states of the radical cation. The TPES measured for the CH2I radical shows several structures that correspond to the photoionization into excited states of the radical cation and are fully assigned on the basis of the calculations. The TPES obtained for the CHI is characterized by a broad structure peaking at 9.335 eV, which could be due to the photoionization from both the singlet and the triplet states and into one or more electronic states of the cation. A vibrational progression is clearly observed in the TPES for the CI radical and a frequency for the C−I stretching mode of 760 ± 60 cm−1 characterizing the CI+ electronic ground state has been extracted., Ministerio de Ciencia, Innovación y Universidades (España), Consejo Superior de Investigaciones Científicas (España), French Agencie Nationale de la Recherche, Depto. de Química Física, Fac. de Ciencias Químicas, TRUE, pub

A prompt site-specific hydrogen-atom elimination from the a-carbon atom (Ca) has been recently reported to occur in the photodissociation of ethyl radicals following excitation at 201 nm [Chicharro et al., Chem. Sci., 2019, 10, 6494]. Such pathway was accessed by means of an initial ro-vibrational energy characterizing the radicals produced by in situ photolysis of a precursor. Here, we present experimental evidence of a similar dynamics in a series of alkyl radicals (C2H5, n-C3H7, n-C4H9, and i-C3H7) containing the same reaction coordinate, but different extended structures. The main requirements for the site-specific mechanism in the studied radicals, namely a rather high content of internal energy prior to dissociation and the participation of vibrational promoting modes, is discussed in terms of the chemical structure of the radicals. The methyl deformation mode in all alkyl radicals along with the CH bending motion in i-C3H7 appear to promote this fast H-atom elimination channel. The photodissociation dynamics of the simplest unsaturated alkyl radical, the vinyl radical (C2H3), is also discussed, showing no signal of site-specific fast H-atom elimination. The results are complemented with high-level ab initio electronic structure calculations of potential energy curves of the vinyl radical, which are compared with those previously reported for the ethyl radical.

The C–C bond dissociation channel of the ethyl (CH3–CH2) radical is investigated at 197.4, 199.98 and 201 nm by velocity map imaging and resonance enhanced multiphoton ionisation (REMPI) detection of the produced CH3 fragments. On the light of high-level ab initio calculations performed in this work, up to four dissociation pathways are identified leading, respectively, to ground state CH3 radicals in correlation with CH2 fragments in the first four electronic states. The major pathway is associated with threshold dissociation over an exit barrier located at the point of a conical intersection between the initially populated 3pA state and a dissociative valence state. As discussed in the text, the concomitance of both features opens the narrow window in which C–C bond dissociation occurs for ethyl and, presumably, for other similar alkyl radicals.

The nonadiabatic photodissociation dynamics of the CH$_3$ (and CD$_3$) radical from the 3p$_z$ and 3s Rydberg states is investigated by applying a one-dimensional (1D) wave packet model that uses recently calculated ab initio 1D electronic potential-energy curves and nonadiabatic couplings. Calculated predissociation lifetimes are found to be too long as compared to the experimental ones. The 1D dynamical model, however, is able to predict qualitatively and explain the fragmentation mechanisms that produce the hydrogen-fragment translational energy distributions (TED) measured experimentally for the ground vibrational resonance of the 3p$_z$ and 3s Rydberg states (CH$_3$($v=0$, 3p$_z$) and CH$_3$($v=0$, 3s)). The CH$_3$($v=0$, 3p$_z$) TED found experimentally displays a rather large energy spreading, while the experimental CH$_3$($v=0$, 3s) TED is remarkably more localized in energy. The present model also predicts a widely spread CH$_3$($v=0$, 3p$_z$) TED, produced by a complex dissociation mechanism which involves predissociation to one dissociative valence state through a nonadiabatic coupling, as well as transfer of population to a second valence state through three conical intersections. Also in agreement with experiment, the model predicts a rather localized CH$_3$($v=0$, 3s) TED because the conical intersections no longer operate in this photodissociation process, and predissociation occurs only into a single valence state. Another complex dissociation mechanism is predicted by the model for initial CH$_3$($v>0$, 3s) and CD$_3$($v>0$, 3s) resonances. In this case the mechanism is gradually activated, as vibrational excitation increases, by the interplay between the two nonadiabatic couplings connecting the 3s and 3p$_{x,y}$ Rydberg states with the dissociative $^2$A$_1$ valence state, and produces complex TEDs with signals from several resonances of both 3s and 3p$_{x,y}$. Thus the present 1D quantum model reveals a rich photodissociation dynamics of methyl

VUV photoionization of the CHnI radicals (with n = 0, 1, and 2) is investigated by means of synchrotron radiation coupled with a double imaging photoion-photoelectron coincidence spectrometer. Photoionization efficiencies and threshold photoelectron spectra (TPES) for photon energies ranging between 9.2 and 12.0 eV are reported. An adiabatic ionization energy (AIE) of 8.334 ± 0.005 eV is obtained for CH2I, which is in good agreement with previous results [8.333 ± 0.015 eV, Sztáray J. Chem. Phys. 2017, 147, 013944], while for CI an AIE of 8.374 ± 0.005 eV is measured for the first time and a value of ∼8.8 eV is estimated for CHI. Ab initio calculations have been carried out for the ground state of the CH2I radical and for the ground state and excited states of the radical cation CH2I+, including potential energy curves along the C–I coordinate. Franck–Condon factors are calculated for transitions from the CH2I(X̃2B1) ground state of the neutral radical to the ground state and excited states of the radical cation. The TPES measured for the CH2I radical shows several structures that correspond to the photoionization into excited states of the radical cation and are fully assigned on the basis of the calculations. The TPES obtained for the CHI is characterized by a broad structure peaking at 9.335 eV, which could be due to the photoionization from both the singlet and the triplet states and into one or more electronic states of the cation. A vibrational progression is clearly observed in the TPES for the CI radical and a frequency for the C–I stretching mode of 760 ± 60 cm–1 characterizing the CI+ electronic ground state has been extracted.

Clocking of electronically and vibrationally state-resolved channels of the fast photodissociation of CH3I in the A-band is re-examined in a combined experimental and theoretical study. Experimentally, a femtosecond pump-probe scheme is employed in the modality of resonant probing by resonance enhanced multiphoton ionization (REMPI) of the methyl fragment in different vibrational states and detection through fragment velocity map ion (VMI) imaging as a function of the time delay. We revisit excitation to the center of the A-band at 268 nm and report new results for excitation to the blue of the band center at 243 nm. Theoretically, two approaches have been employed to shed light into the observations: first, a reduced dimensionality 4D nonadiabatic wavepacket calculation using the potential energy surfaces by Xie et al. [J. Phys. Chem. A 104, 1009 (2000)]; and second, a full dimension 9D trajectory surface-hopping calculation on the same potential energy surfaces, including the quantization of vibrational states of the methyl product. In addition, high level ab initio electronic structure calculations have been carried out to describe the CH3 3pz Rydberg state involved in the (2 + 1) REMPI probing process, as a function of the carbon-iodine (C–I) distance. A general qualitative agreement is obtained between experiment and theory, but the effect of methyl vibrational excitation in the umbrella mode on the clocking times is not well reproduced. The theoretical results reveal that no significant effect on the state-resolved appearance times is exerted by the nonadiabatic crossing through the conical intersection present in the first absorption band. The vibrationally state resolved clocking times observed experimentally can be rationalized when the (2 + 1) REMPI probing process is considered. None of the other probing methods applied thus far, i.e., multiphoton ionization photoelectron spectroscopy, soft X-ray inner-shell photoelectron spectroscopy, VUV single-photon i, Ministerio de Ciencia, Invocación y Universidades (España), Ministerio de Economía y Competitividad (España), Universidad Complutense de Madrid, Depto. de Química Física, Fac. de Ciencias Químicas, TRUE, pub

It is demonstrated both numerically and mathematically that the dynamical behavior of an isolated resonance state -which comprises the resonance decay lifetime and the asymptotic fragment state distribution produced upon resonance decay- can be extensively controlled by means of quantum interference induced by a laser field in the weak-field regime. The control scheme applied is designed to induce interference between amplitudes excited at two different energies of the resonance lineshape, namely the resonance energy and another energy. The scheme exploits the resonance property of possessing a nonzero energy width, which makes possible that a resonance may interfere with itself, and thus allows interference between the amplitudes excited at the two energies of the resonance width. Application of this scheme opens the possibility of a universal control of both the duration and the fragment product distribution outcome of any resonance-mediated molecular process.

Clocking of electronically and vibrationally state-resolved channels of the fast photodissociation of CH3I in the A-band is re-examined in a combined experimental and theoretical study. Experimentally, a femtosecond pump-probe scheme is employed in the modality of resonant probing by resonance enhanced multiphoton ionization (REMPI) of the methyl fragment in different vibrational states and detection through fragment velocity map ion (VMI) imaging as a function of the time delay. We revisit excitation to the center of the A-band at 268 nm and report new results for excitation to the blue of the band center at 243 nm. Theoretically, two approaches have been employed to shed light into the observations: first, a reduced dimensionality 4D nonadiabatic wavepacket calculation using the potential energy surfaces by Xie et al. [J. Phys. Chem. A 104, 1009 (2000)]; and second, a full dimension 9D trajectory surface-hopping calculation on the same potential energy surfaces, including the quantization of vibrational states of the methyl product. In addition, high level ab initio electronic structure calculations have been carried out to describe the CH3 3pz Rydberg state involved in the (2 + 1) REMPI probing process, as a function of the carbon-iodine (C¿I) distance. A general qualitative agreement is obtained between experiment and theory, but the effect of methyl vibrational excitation in the umbrella mode on the clocking times is not well reproduced. The theoretical results reveal that no significant effect on the state-resolved appearance times is exerted by the nonadiabatic crossing through the conical intersection present in the first absorption band. The vibrationally state resolved clocking times observed experimentally can be rationalized when the (2 + 1) REMPI probing process is considered. None of the other probing methods applied thus far, i.e., multiphoton ionization photoelectron spectroscopy, soft X-ray innershell photoelectron spectroscopy, VUV single-photon io

The photochemistry of the ethyl radical following excitation to the 3p Rydberg state is investigated in a joint experimental and theoretical study. Velocity map images for hydrogen atoms detected from photoexcited isotopologues CH3CH2, CH3CD2 and CD3CH2 at 201 nm, are discussed along with high-level ab initio electronic structure calculations of potential energy curves and non-adiabatic coupling matrix elements (NACME). A novel mechanism governed by a conical intersection allowing prompt site-specific hydrogen-atom elimination is presented and discussed. For this mechanism to occur, an initial rovibrational excitation is allocated to the radical permitting to access this reaction pathway and thus to control the ethyl photochemistry. While hydrogen-atom elimination from cold ethyl radicals occurs through internal conversion into lower electronic states followed by slow statistical dissociation, prompt site-specific Ca elimination into CH3CH + H, occurring through a fast non-adiabatic crossing to a valence bound state followed by dissociation through a conical intersection, is accessed by means of an initial ro-vibrational energy content into the radical. The role of a particularly effective vibrational promoting mode in this prompt photochemical reaction pathway is discussed., Depto. de Química Física, Fac. de Ciencias Químicas, TRUE, pub

The photochemistry of the ethyl radical following excitation to the 3p Rydberg state is investigated in a joint experimental and theoretical study. Velocity map images for hydrogen atoms detected from photoexcited isotopologues CH3CH2, CH3CD2 and CD3CH2 at 201 nm, are discussed along with high-level ab initio electronic structure calculations of potential energy curves and non-adiabatic coupling matrix elements (NACME). A novel mechanism governed by a conical intersection allowing prompt site-specific hydrogen-atom elimination is presented and discussed. For this mechanism to occur, an initial rovibrational excitation is allocated to the radical permitting to access this reaction pathway and thus to control the ethyl photochemistry. While hydrogen-atom elimination from cold ethyl radicals occurs through internal conversion into lower electronic states followed by slow statistical dissociation, prompt site-specific Ca elimination into CH3CH + H, occurring through a fast non-adiabatic crossing to a valence bound state followed by dissociation through a conical intersection, is accessed by means of an initial ro-vibrational energy content into the radical. The role of a particularly effective vibrational promoting mode in this prompt photochemical reaction pathway is discussed.

A weak-field coherent control scheme is applied in order to enhance the decay lifetime of a superposition of overlapping resonance states. The scheme uses a pump laser field consisting of two pulses delayed in time, each of them exciting a different energy at which several resonances of the Ne-Br$_2(B)$ complex overlap. Simultaneous excitation of these two energies induces interference between the overlapping resonances, which causes an enhancement of the lifetime of the superposition created. By varying the delay time between the pulses, the mechanism of resonance interference can be controlled and optimized to achieve a maximum lifetime enhancement. The optimal delay time between pulses leading to maximum superposition lifetime can be quantitatively predicted with a simple law. The effect of the interference mechanism on the lifetime enhancement is investigated. It is found that interference induces a transfer of amplitude between the different resonances back and forth, which delays significantly the natural resonance decay, increasing the global lifetime of the superposition. Due to the simplicity of the control scheme, a wide applicability is envisioned.

The photodissociation dynamics of the ethyl radical following excitation into the 3s and 3p Rydbergstates are revisited in a joint experimental and theoretical study. Two different methods to produce theethyl radical, pyrolysis andin situphotolysis, are employed in order to modify the initial ro-vibrationalenergy distribution characterizing the ethyl radical beam. H-atom velocity map images followingexcitation of the radical at 243 nm and at 201 nm are presented and discussed along withab initiopotential energy curves focussing on the bridgedC2vgeometry. The reported results show that thedynamics following excitation to the 3s Rydberg state is insensitive to the initial internal energy of theparent radical, in contrast to the dynamics on the 3p Rydberg state, which is strongly modified. The roleof the bridgedC2vgeometry on both photodynamics is highlighted and discussed.