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Multiphoton ionization (MPI) of alkyl iodides (RI, R = CnH2n+1, n = 1-4) has been investigated with femtosecond laser pulses centered at 800 and 400 nm along with photoelectron imaging detection. In addition, the ultraviolet (UV)-vacuum ultraviolet (VUV) absorption spectra of gas-phase RIs have been measured in the photon energy range of 5-11 eV using the VUV Fourier transform spectrometer at the VUV DESIRS beamline of the synchrotron SOLEIL facility. The use of high-laser-field strengths in matter-radiation interaction generates highly non-linear phenomena, such as the Stark shift effect, which distorts the potential energy surfaces of molecules by varying both the energy of electronic and rovibrational states and their ionization energies. The Stark shift can then generate resonances between intermediate states and an integer number of laser photons of a given wavelength, which are commonly known as Freeman resonances. Here, we study how the molecular structure of linear and branched alkyl iodides affects the UV-VUV absorption spectrum, the MPI process, and the generation of Freeman resonances. The obtained results reveal a dominant resonance in the experiments at 800 nm, which counter-intuitively appears at the same photoelectron kinetic energy in the whole alkyl iodide series. The ionization pathways of this resonance strongly involve the 6p(2E3/2) Rydberg state with different degrees of vibrational excitation, revealing an energy compensation effect as the R-chain complexity increases., (© 2023 Author(s). Published under an exclusive license by AIP Publishing.)

Multiphoton ionization (MPI) of methyl iodide, CH 3 I, has been investigated with the photoelectron imaging (PEI) technique, using high intensity femtosecond laser pulses at different central wavelengths. The use of high laser field strengths alters the way in which matter-radiation interaction takes place. This generates highly nonlinear phenomena, among which we can highlight the Stark shift effect. It can distort the potential energy surfaces of atoms and molecules, varying both the energy of electronic and rovibrational states of these systems and their ionization potentials. In this way, the Stark shift can generate resonances between intermediate states and an integer number of laser photons of a given wavelength, which would be absent in the low intensity regime. The main purpose of this work is the generation, detection and characterization of resonances produced by the Stark shift, commonly known as Freeman resonances, induced by multiphoton ionization of gas-phase CH 3 I at different laser wavelengths. The results obtained reveal that a multitude of resonances are induced in the ionization of CH 3 I in the range of intensities employed, involving several Rydberg states. Ionization pathways associated with different degrees of vibrational excitation in both the intermediate states and the molecular cation generated in each of the experiments are proposed.

The photochemical dynamics of double-bond-containing hydrocarbons is exemplified by the smallest alkenes, ethylene and butadiene. Chemical substituents can alter both decay timescales and photoproducts through a combination of inertial effects due to substituent mass, steric effects due to substituent size, and electronic (or potential) effects due to perturbative changes to the electronic potential energy surface. Here, we demonstrate the interplay of different substituent effects on 1,3-butadiene and its methylated derivatives using a combination of ab initio simulation of nonadiabatic dynamics and time-resolved photoelectron spectroscopy. The purely inertial effects of methyl substitution are simulated through the use of mass 15 "heavy-hydrogen" atoms. As expected from both inertial and electronic influences, the excited-state dynamics is dominated by pyramidalization at the unsubstituted carbon sites. Although the electronic effects of methyl group substitution are weak, they alter both decay timescales and branching ratios by influencing the initial path taken by the excited wavepacket following photoexcitation.

Clocking of electronically and vibrationally state-resolved channels of the fast photodissociation of CH 3 I 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 CH 3 3p z 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 ionization, and XUV core-to-valence transient absorption spectroscopy, have been able to provide quantum state-resolved (vibrational) clocking times. More experiments would be needed to disentangle the fine details in the clocking times and dissociation dynamics arising from the detection of specific quantum-states of the molecular fragments.

Femtosecond time-resolved velocity map ion imaging experiments are reported on the second absorption band (B-band) of ethyl iodide at 201.19 and 200.08 nm, corresponding to the 000 and 1810 transitions, i.e., the origin of the band and the first most intense vibronic state assigned to one quantum of excitation in the methyl torsion mode. Electronic predissociation lifetimes and the temporal evolution of the anisotropy have been determined by time-resolved resonance-enhanced multiphoton ionization of iodine and ethyl fragment images. A shorter lifetime measured at the origin of the band in comparison with methyl iodide indicates that predissociation in ethyl iodide is more favorable due to a stronger coupling between the initial Rydberg state and the valence repulsive state correlating with the dissociation fragments. Moreover, vibrational activity in the methyl torsion in the Rydberg state seems to enhance the probability of transfer of population to the valence repulsive state leading to a faster dissociation. The perpendicular character of the transition at early times and the loss of anisotropy as a function of time have been determined from the time-resolved angular distributions of the iodine and ethyl ion images. The initial anisotropy value is consistent with a purely perpendicular transition compatible with the excitation of the [6A'', 7A'] states with a minor parallel component to the C-I bond. The loss of initial anisotropy over time highlights the parent molecular rotation during predissociation and is compatible with a rotational temperature of the parent molecule of 100 K.

Coulomb explosion imaging is proposed as a method to directly map the presence of conical intersections encountered by a propagating wave packet in a molecular system. The case of choice is the nonadiabatic coupling between two dissociative surfaces in the methyl iodide molecule, probed by Coulomb explosion with short, intense near-infrared pulses causing multiple ionization. On-the-fly multidimensional trajectory calculations with surface hopping using perturbation theory and including spin-orbit coupling are performed to visualize the dynamics through the conical intersection and compare with experimental results. The possibilities and limitations of the technique are examined and discussed.

A comparative study of the ultrafast photodissociation dynamics of the dihalomethanes CH2ICl and CH2BrI has been carried out at 268 nm, around the maximum of the first absorption band, employing femtosecond velocity map ion imaging in conjunction with high level ab initio electronic structure calculations and full dimension on-the-fly trajectory calculations including surface hopping. Total translational energy distributions and angular distributions of the iodine fragments as well as reaction times for the C-I bond cleavage are presented and discussed along with the computed absorption spectra, potential energy curves and trajectories. The revealed dynamics is mainly governed by absorption to the 5A' state for CH2BrI while two dissociation pathways, through the 4A' or 5A' states, are in competition for CH2lCI. An anchor effect due to the substituent halogen atom (Br or Cl), which implies significant rotational motion of the dissociating molecule, characterizes the photodissociation in both dihalomethanes and leads to a remarkable rotational energy of the radical co-fragment. This energy flux into the internal degrees of freedom of the molecules is the main key factor governing the real time reaction dynamics.

The notion that strong laser light can intervene and modify the dynamical processes of matter has been demonstrated and exploited both in gas and condensed phases. The central objective of laser control schemes has been the modification of branching ratios in chemical processes, under the philosophy that conveniently tailored light can steer the dynamics of a chemical mechanism towards desired targets. Less explored is the role that strong laser control can play on chemical stereodynamics, i.e. the angular distribution of the products of a chemical reaction in space. This work demonstrates for the case of methyl iodide that when a molecular bond breaking process takes place in the presence of an intense infrared laser field, its stereodynamics is profoundly affected, and that the intensity of this laser field can be used as an external knob to control it.

This Letter presents an experimental and theoretical study of femtosecond time-resolved vector correlations in methyl iodide (CH 3 I) electronic predissociation via the second absorption B-band at 201.2 nm. The time evolution of the phenomenological anisotropy parameters β l was determined from time-resolved photofragment angular distributions obtained by means of the femtosecond laser pump-probe technique coupled with velocity map imaging detection of vibrational ground-state CH 3 (ν = 0) fragments and spin-orbit excited I*( 2 P 1/2 ) atoms. Theoretical interpretation of the experimental results was performed on the basis of a fitting procedure using quasiclassical theory, which elucidates vector correlations in photodissociation of symmetric top molecules. The results of the fitting are in very good agreement with the experimental data and demonstrate the important role of molecular excited-state lifetimes, parent molecule and methyl fragment rotations, and methyl fragment angular momentum alignment on the time-dependent electronic predissociation dynamics.

Experiments aimed at understanding ultrafast molecular processes are now routine, and the notion that external laser fields can constitute an additional reagent is also well established. The possibility of externally controlling a reaction with radiation increases immensely when its intensity is sufficiently high to distort the potential energy surfaces at which chemists conceptualize reactions take place. Here we explore the transition from the weak- to the strong-field regimes of laser control for the dissociation of a polyatomic molecule, methyl iodide. The control over the yield of the photodissociation reaction proceeds through the creation of a light-induced conical intersection. The control of the velocity of the product fragments requires external fields with both high intensities and short durations. This is because the mechanism by which control is exerted involves modulating the potentials around the light-induced conical intersection, that is, creating light-induced potentials.

The correlation between chemical structure and dynamics has been explored in a series of molecules with increasing structural complexity in order to investigate its influence on bond cleavage reaction times in a photodissociation event. Femtosecond time-resolved velocity map imaging spectroscopy reveals specificity of the ultrafast carbon-iodine (C-I) bond breakage for a series of linear (unbranched) and branched alkyl iodides, due to the interplay between the pure reaction coordinate and the rest of the degrees of freedom associated with the molecular structure details. Full-dimension time-resolved dynamics calculations support the experimental evidence and provide insight into the structure-dynamics relationship to understand structural control on time-resolved reactivity.

Strong field control scenarios are investigated in the CH3I predissociation dynamics at the origin of the second absorption B-band, in which state-selective electronic predissociation occurs through the crossing with a valence dissociative state. Dynamic Stark control (DSC) and pump-dump strategies are shown capable of altering both the predissociation lifetime and the product branching ratio.

The Coulomb explosion of CH(3)I in an intense (10-100 TW cm(-2)), ultrashort (50 fs) and nonresonant (804 nm) laser field has been studied experimentally and justified theoretically. Ion images have been recorded using the velocity map imaging (VMI) technique for different singly and multiply charged ion fragments, CH(3)(p+) (p = 1) and I(q+) (q ≤ 3), arising from different Coulomb explosion channels. The fragment kinetic energy distributions obtained from the measured images for these ion fragments show significantly lower energies than those expected considering only Coulomb repulsion forces. The experimental results have been rationalized in terms of one-dimensional wave packet calculations on ab initio potential energy curves of the different multiply charged species. The calculations reveal the existence of a potential energy barrier due to a bound minimum in the potential energy curve of the CH(3)I(2+) species and a strong stabilization with respect to the pure Coulombic repulsion for the higher charged CH(3)I(n+) (n = 3, 4) species., (© 2011 American Chemical Society)

Femtosecond time-resolved velocity map imaging experiments are reported on several vibronic levels of the second absorption band (B-band) of CH(3)I, including vibrational excitation in the ν(2) and ν(3) modes of the bound (3)R(1)(E) Rydberg state. Specific predissociation lifetimes have been determined for the 2(0)(1) and 3(0)(1) vibronic levels from measurements of time-resolved I*((2)P(1/2)) and CH(3) fragment images, parent decay, and photoelectron images obtained through both resonant and non-resonant multiphoton ionization. The results are compared with our previously reported predissociation lifetime measurements for the band origin 0(0) (0) [Gitzinger et al., J. Chem. Phys. 132, 234313 (2010)]. The result, previously reported in the literature, where vibrational excitation to the C-I stretching mode (ν(3)) of the CH(3)I (3)R(1)(E) Rydberg state yields a predissociation lifetime about four times slower than that corresponding to the vibrationless state, whereas predissociation is twice faster if the vibrational excitation is to the umbrella mode (ν(2)), is confirmed in the present experiments. In addition to the specific vibrational state lifetimes, which were found to be 0.85 ± 0.04 ps and 4.34 ± 0.13 ps for the 2(0)(1) and 3(0)(1) vibronic levels, respectively, the time evolution of the fragment anisotropy and the vibrational activity of the CH(3) fragment are presented. Additional striking results found in the present work are the evidence of ground state I((2)P(3/2)) fragment production when excitation is produced specifically to the 3(0)(1) vibronic level, which is attributed to predissociation via the A-band (1)Q(1) potential energy surface, and the indication of a fast adiabatic photodissociation process through the repulsive A-band (3)A(1)(4E) state, after direct absorption to this state, competing with absorption to the 3(0)(1) vibronic level of the (3)R(1)(E) Rydberg state of the B-band., (© 2012 American Institute of Physics)

The ultrafast relaxation of 1-iodonaphthalene, with particular attention to the dissociation channels, has been studied by time-resolved femtosecond pump-probe mass spectrometry following excitation at 267 and 317 nm. The measured transients for the parent ion and the isobaric fragments, iodine and naphthyl radical, show complex decay profiles with up to four lifetimes in the femto-picosecond time scales. The transients are interpreted as the result of parallel relaxation of the simultaneously excited n sigma* and pi pi* states of the molecule. While the former leads to dissociation in about 400 fs, the latter converts to lower energy pi pi* singlet states at an ultrafast rate (24 fs) followed by intersystem crossing to nearby pi pi* triplet states.

A femtosecond pump-probe experiment, coupled with velocity map ion imaging, is reported on the second absorption band (B-band) of CH(3)I. The measurements provide a detailed picture of real-time B-band predissociation in the band origin at 201.2 nm. Several new data are reported. (i) A value of 1.5+/-0.1 ps has been obtained for the lifetime of the excited state, consistent within errors with the only other direct measurement of this quantity [A. P. Baronavski and J. C. Owrutsky, J. Chem. Phys. 108, 3445 (1998)]. (ii) It has been possible to measure the angular character of the transition directly through the observation of fragments appearing early with respect to both predissociation lifetime and molecular rotation. (iii) Vibrational activity in CH(3) has been found, both in the umbrella (nu(2)) and the symmetric stretch (nu(1)) modes, with estimates of relative populations. All these findings constitute a challenge and a test for much-wanted high level ab initio and dynamics calculations in this energy region.