Dissociative photoionization of acetaldehyde in the 10.2-19.5 eV VUV range.

The valence-shell dissociative photoionization of acetaldehyde has been investigated by means of the photoion photoelectron coincidence technique in conjunction with tuneable synchrotron radiation. The experimental results consist of threshold photoelectron spectra for the parent ion and for each fragment ion in the 10.2-19.5 eV photon energy range, along with (ion, e) kinetic energy coincidence diagrams obtained from measurements at fixed photon energies. The results are complemented by high-level ab initio calculations of potential energy curves as a function of the C-H bond distance. The nudged elastic band (NEB) method has been employed to connect the parent ion Franck-Condon region to the formation of the HCO+, CH3+ and CH4+ ion fragments. Appearance energies have been determined for six fragment ions with an improved accuracy, including two fragmentation channels, which to the best of our knowledge have not been reported previously, i.e. the formation of CH2CO+, lying at 13.10 ± 0.05 eV, and the formation of CH2+ at 15.1 ± 0.1 eV. Based on both experimental and theoretical results, the dissociation dynamics following ionization of acetaldehyde into the different fragmentation channels are discussed.

Effects of swift heavy ions on metal nanoparticles embedded in silica: Using linearly polarized light to monitor the elongation kinetics.

The elongation of metallic nanoparticles (NPs) embedded in a dielectric matrix after irradiation with swift heavy ions is a phenomenon that has been known for several years. However, the precise mechanism behind this deformation process is still not fully understood, primarily due to the dearth of information during intermediate stages of deformation. In this study, we report the continuation of our previous work [Peña-Rodríguez et al., Sci. Rep. 7(1), 922 (2017)], exploiting the strong dependence of the localized surface plasmon resonance on the aspect ratio of elongated metal NPs to study the elongation kinetics in situ. In situ optical absorption spectra were measured using a polarizing beam splitter to separate the longitudinal and transverse plasmon modes of the anisotropic NPs. Then, the detailed geometrical and compositional parameters were determined from a fit of these spectra. The use of linearly polarized light allowed for a more accurate analysis of the elongation kinetics, particularly useful in the first stages, where longitudinal and transverse modes overlap.

Imaging the photodissociation dynamics of internally excited ethyl radicals from high Rydberg states.

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 Cα-H/D and Cß-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/D-atoms 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.

Photodissociation dynamics of methylamine in the blue edge of the A-band. I. The H-atom elimination channel.

The photodissociation dynamics of methylamine (CH3NH2) upon excitation in the blue edge of the first absorption A-band, in the 198-203 nm range, are investigated by means of nanosecond pump-probe laser pulses and velocity map imaging combined with H(2S)-atom detection through resonance enhanced multiphoton ionization. The images and corresponding translational energy distributions for the H-atoms produced show three different contributions associated with three reaction pathways. The experimental results are complemented by high-level ab initio calculations. The potential energy curves computed as a function of the N-H and C-H bond distances allow us to draw a picture of the different mechanisms. Major dissociation occurs through N-H bond cleavage and it is triggered by an initial geometrical change, i.e., from a pyramidal configuration of the C-NH2 with respect to the N atom to a planar geometry. The molecule is then driven into a conical intersection (CI) seam where three outcomes can take place: first, threshold dissociation into the second dissociation limit, associated with the formation of CH3NH(Ã), is observed; second, direct dissociation after passage through the CI leading to the formation of ground state products; and third, internal conversion into the ground state well in advance to dissociation. While the two last pathways were previously reported at a variety of wavelengths in the 203-240 nm range, the former had not been observed before to the best of our knowledge. The role of the CI and the presence of an exit barrier in the excited state, which modify the dynamics leading the two last mechanisms, are discussed considering the different excitation energies used.

Photodissociation dynamics of methylamine in the blue edge of the A-band. II. The NH2 + CH3 channel.

The photodissociation dynamics leading to the C-N bond cleavage in methylamine (CH3NH2) are investigated upon photoexcitation in the blue edge of the first absorption A-band, in the 198-204 nm range. Velocity map images of the generated methyl (CH3) fragment detected in specific vibrational modes, i.e., ν = 0, ν1 = 1, and ν2 = 1, through resonance enhanced multiphoton ionization, are presented along with the corresponding translational energy distributions and the angular analysis. The experimental results are complemented by high-level ab initio calculations of potential energy curves as a function of the C-N bond distance. While a similar single Boltzmann-type contribution is observed in all the translational energy distributions measured, the speed-dependent anisotropy parameter obtained through the angular analysis reveals the presence of two different mechanisms. Prompt dissociation through the conical intersection between the Ã1A' first excited state and the ground state located in the exit channel is, indeed, revealed as a minor channel. In contrast, slow dissociation on the ground state, presumably from frustrated N-H bond cleavage trajectories, constitutes the major reaction pathway leading to the methyl formation.

Stark control of multiphoton ionization through Freeman resonances in alkyl iodides.

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.

Wavelength dependence of the multiphoton ionization of CH3I by intense femtosecond laser pulses through Freeman resonances.

Multiphoton ionization (MPI) of methyl iodide, CH3I, 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 CH3I at different laser wavelengths. The results obtained reveal that a multitude of resonances are induced in the ionization of CH3I 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.

Imaging the Photodissociation Dynamics and Fragment Alignment of CH2BrI at 193 nm.

The photodissociation dynamics and photofragment alignment of bromoiodomethane (CH2BrI) have been studied at 193 nm using a double experimental and theoretical approach. In addition, the ultraviolet (UV)-vacuum ultraviolet (VUV) absorption spectrum of gas phase CH2BrI has been measured in the photon energy range of 5-11 eV using the VUV Fourier transform spectrometer (FTS) at the VUV beamline DESIRS of the synchrotron SOLEIL facility. The slice imaging technique in combination with resonance enhanced multiphoton ionization (REMPI) detection of the Br(2PJ) and I(2PJ) (with J = 3/2 and 1/2 for Br/I and Br*/I*, respectively) atomic photofragments have been used to produce experimental translational energy and angular distributions, which were analyzed to deliver, on one hand, the partitioning of the available energy among the different degrees-of-freedom of the photofragments and, on the other, the photofragment polarization in terms of aqk(p) alignment parameters. The experimental measurements were rationalized in terms of high-level ab initio calculations of vertical excitation energies, transition dipole moments and potential energy curves (PECs) along different reaction coordinates to provide a complete picture of the photodissociation dynamics. The results indicate that for excitation at 193 nm, prompt C-X cleavage (with X being either halogen atom, Br or I) competes with fast internal conversion and consequent stochastic dissociation in lower electronic states. In the case of the CH2Br + I(2P3/2)/I*(2P1/2) channels, the dynamics are greatly biased toward the stochastic dissociation process due to both the particular PECs landscape and the unfavored excitation of the CH2BrI ensemble with respect to the C-I molecular axis at this excitation energy. The ab initio PECs provide a tentative path for the fast dissociation process in either case. For the C-Br bond breakage, excitation to the 13A' electronic state and predissociation through the 11A'/11A″ or 12A'/12A″ states, leading to direct dissociation through the 10A'/9A″ states, appear as the most consistent dynamics. For the C-I channel, predissociation does not become a reliable possibility and a fast internal conversion may precede dissociation through the repulsive 6A'/6A″ and 4A'/4A″ states. The large content of rotational and vibrational excitation of the polyatomic cofragments is justified through the soft impulsive model and the geometrical changes produced along the dissociation pathway. Strikingly, the aqk(p) alignment parameters obtained for the Br(2P3/2) and I(2P3/2) photoproducts indicate that the rotational angular momentum of the CH2X (X = I or Br) cofragment appears highly constrained along the recoil direction. Finally, this work presents a highly plausible explanation for the branching ratio of secondary dissociation processes in the photodynamics of CH2BrI at 193 nm.

Conical intersection and coherent vibrational dynamics in alkyl iodides captured by attosecond transient absorption spectroscopy.

The photodissociation dynamics of alkyl iodides along the C-I bond are captured by attosecond extreme-ultraviolet (XUV) transient absorption spectroscopy employing resonant ∼20 fs UV pump pulses. The methodology of previous experiments on CH3I [Chang et al., J. Chem. Phys. 154, 234301 (2021)] is extended to the investigation of a C-I bond-breaking reaction in the dissociative A-band of C2H5I, i-C3H7I, and t-C4H9I. Probing iodine 4d core-to-valence transitions in the XUV enables one to map wave packet bifurcation at a conical intersection in the A-band as well as coherent vibrations in the ground state of the parent molecules. Analysis of spectroscopic bifurcation signatures yields conical intersection crossing times of 15 ± 4 fs for CH3I, 14 ± 5 fs for C2H5I, and 24 ± 4 fs for i-C3H7I and t-C4H9I, respectively. Observations of coherent vibrations, resulting from a projection of A-band structural dynamics onto the ground state by resonant impulsive stimulated Raman scattering, indirectly reveal multimode C-I stretch and CCI bend vibrations in the A-bands of C2H5I, i-C3H7I, and t-C4H9I.

Site-specific hydrogen-atom elimination in photoexcited alkyl radicals.

A prompt site-specific hydrogen-atom elimination from the α-carbon atom (Cα) 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.

Threshold Photoelectron Spectroscopy of the CH2I, CHI, and CI Radicals.

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.

Signature of a conical intersection in the dissociative photoionization of formaldehyde.

The valence-shell photoionization of formaldehyde is investigated by means of combining Photo-Electron Photo-Ion COincidence (PEPICO) experiments and ab initio calculations. The formation of three ion fragments: HCO+, CO+ and H, via dissociative photoionization following excitation at 17 eV is discussed. The experimental results consisting of electron-ion kinetic energy correlation diagrams for the corresponding coincident events, i.e. (HCO+, e-), (CO+, e-) and (H, e-), as well as the fragment abundance as a function of the binding energy, are complemented by high level electronic structure calculations including potential energy curves and on-the-fly trajectories. The results are consistent with a main relaxation process via internal conversion into rovibrationally excited levels of the H2CO+ ground state, followed by statistical dissociation, preferentially into HCO+. The analysis of the experimental results reveals nevertheless the signature of a conical intersection controlling the dynamics and favoring dissociation into the molecular channel, CO+ + H2. In addition, the minor formation of the H ion is suggested to occur through a roaming pathway on the cation excited state.

Velocity map imaging study of the photodissociation dynamics of the allyl radical.

The photodissociation of the allyl radical (CH2[double bond, length as m-dash]CH-CH2˙) following excitation between 216 and 243 nm has been investigated employing velocity map imaging in conjunction with resonance enhanced multiphoton ionization to detect the hydrogen atom and CH3(ν = 0) produced. The translational energy distributions for the two fragments are reported and analyzed along with the corresponding fragment ion angular distributions. The results are discussed in terms of the different reactions pathways characterizing the hydrogen atom elimination and the minor methyl formation. On one hand, the angular analysis provides evidence of an additional mechanism, not reported before, leading to prompt dissociation and fast hydrogen atoms. On the other hand, the methyl elimination channel has been characterized as a function of the excitation energy and the contribution of three reaction pathways: single 1,3-hydrogen shift, double 1,2-hydrogen shift and through the formation of vinylidene have been discussed. Contrary to previous predictions, the vinylidene channel, which plays a significant role at lower energies, seems to vanish following excitation on the E[combining tilde]2B1(3px) excited state at λ≤ 230 nm.

Substituent effects on nonadiabatic excited state dynamics: Inertial, steric, and electronic effects in methylated butadienes.

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.

Femtochemistry under scrutiny: Clocking state-resolved channels in the photodissociation of CH3I in the A-band.

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 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 predissociation dynamics of ethyl iodide in the B-band.

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.

Dynamics of the photodissociation of ethyl iodide from the origin of the B band. A slice imaging study.

The photodissociation dynamics and stereodynamics of ethyl iodide from the origin of the second absorption B-band have been investigated combining pulsed slicFe imaging with resonance enhanced multiphoton ionization (REMPI) detection of all fragments, I(2P3/2), I*(2P1/2) and C2H5. The I*(2P1/2) atom action spectrum recorded as a function of the excitation wavelength permits one to identify and select the 0 origin of this band at 201.19 nm (49 704 cm-1). Translational energy distributions and angular distributions for all fragments and semiclassical Dixon's bipolar moments for the C2H5 fragment are presented and discussed along with high-level ab initio calculations of potential energy curves as a function of the C-I distance. A predissociative mechanism governs the dynamics where in a first step a bound Rydberg state corresponding to the 5pπI→ 6sI transition is populated by the 201.19 nm-photon absorption. A curve crossing with a repulsive state located within the Franck-Condon geometry leads to direct dissociation into the major channel C2H5 + I*(2P1/2). A small amount of I(2P3/2) atoms is nevertheless observed and presumably attributed to a second curve crossing with a repulsive state from the A-band.

Threshold photoelectron spectrum of the CH2OO Criegee intermediate.

We present the photoelectron spectroscopy of the simplest Criegee intermediate, CH2OO, close to the first ionization energy. Comparison with existing theoretical data yields the experimental adiabatic ionization energy and provides a benchmark for theoretical studies on larger Criegee intermediates, which play an important role in the ozonolysis of alkenes.

The 3s versus 3p Rydberg state photodissociation dynamics of the ethyl radical.

The photodissociation dynamics of the ethyl radical following excitation into the 3s and 3p Rydberg states are revisited in a joint experimental and theoretical study. Two different methods to produce the ethyl radical, pyrolysis and in situ photolysis, are employed in order to modify the initial ro-vibrational energy distribution characterizing the ethyl radical beam. H-atom velocity map images following excitation of the radical at 243 nm and at 201 nm are presented and discussed along with ab initio potential energy curves focussing on the bridged C2v geometry. The reported results show that the dynamics following excitation to the 3s Rydberg state is insensitive to the initial internal energy of the parent radical, in contrast to the dynamics on the 3p Rydberg state, which is strongly modified. The role of the bridged C2v geometry on both photodynamics is highlighted and discussed.

Photodissociation Dynamics and Stereodynamics of Methyl Mercaptan and Dimethyl Sulfide from the Second Absorption Band at 201 and 210 nm.

The role of promoting and spectator modes vs energy randomization in nonadiabatic dynamics is interrogated in the photodissociation of methyl mercaptan, CH3SH, and dimethyl sulfide, CH3SCH3 or DMS, in the second absorption band. The primary CH3(ν) radicals produced in the dissociation of both systems at 210 nm have been resonantly detected in slice-imaging experiments, and the corresponding translational energy and angular distributions have been obtained. The stereodynamical information provided by Dixon's bipolar moments in conjunction with the energy partitioning among the different degrees of freedom of the primary CH3(ν) products offers a panoramic picture of the photodissociation process of both systems. The remarkable similitude found between the two systems related to both vector correlations and internal energy content of the corresponding counterparts-SH for methyl mercaptan and SCH3 for DMS-indicates that despite the diabaticity of the process, no efficient energy randomization of the available energy takes place. More specifically, only the parent vibrational modes whose participation in the initial absorption step is imposed by the conical intersection-i.e., the promoting modes-are adiabatically preserved during the process, while the rest of the vibrational modes play the spectator role. The results for both molecules at 210 nm are complemented with experiments carried out for DMS at 201 nm to explore the internal mechanism of the conical intersection in different zones of the absorption region.