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Rovibrationally Excited Molecules on the Verge of a Triple Breakdown: Molecular and Roaming Mechanisms in the Photodecomposition of Methyl Formate Andrea Lombardi, Federico Palazzetti, Vincenzo Aquilanti, Hou-Kuan Li, Po-Yu Tsai, Toshio Kasai, and King-Chuen Lin J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.6b00723 • Publication Date (Web): 20 Feb 2016 Downloaded from http://pubs.acs.org on February 21, 2016

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The Journal of Physical Chemistry

Rovibrationally Excited Molecules on the Verge of a Triple Breakdown: Molecular and Roaming Mechanisms in the Photodecomposition of Methyl Formate Andrea Lombardi*,a, Federico Palazzettia, Vincenzo Aquilantia,c,d, Hou-Kuan Lie, Po-Yu. Tsaie†, Toshio Kasaie, King-Chuen Lin**,e aDipartimento

di Chimica Biologia e Biotecnologie, Università di Perugia, Via Elce di Sotto 8, 06123 Perugia, Italy

cIstituto

di Struttura della Materia, Consiglio Nazionale delle Ricerche, Rome, Italy

dInstituto

eDepartment

de Fisica, Universidade Federal da Bahia, Salvador, Brazil of Chemistry, National Taiwan University, Taipei 106, Taiwan

Corresponding Author *Andrea Lombardi, email: [email protected]. ** King-Chuen Lin, email [email protected] Present Addresses †Department of Chemistry, National Chung Hsing University, Taichung, Taiwan.

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ABSTRACT. For the photodissociation of the simplest of esters, methyl formate HCOOCH3, the energy threshold for triple fragmentation into H, CH3O and CO was measured by previous ionimaging experiments at a sequence of wavelengths. The translational energy features of product CO in the ground vibrational level (v = 0) and for selected rotational states were characterized. In this integrated experimental and theoretical approach (i) the focus is at a laser energy barely below that threshold; (ii) Fourier-Transform-Infra-Red emission spectroscopy measurements probe the rovibrational energy deposition in CO(v ) for v > 0 and the emergence of the roaming phenomenon; (iii) accompanying quantum chemical calculations describe the selective rupture of bonds; and (iv) molecular dynamics simulations of dissociation are performed, introducing an approach explicitly involving outcomes from paths originated nonadiabatically through conical intersections. Quantitative information on energy disposal is provided: we found extensive vibrational excitation of CO, while rotational bands are colder and bimodal, due to contributions from direct and roaming modes.

KEYWORDS

Photodissociation,

conical

intersection,

roaming,

FTIR

spectroscopy,

quasiclassical trajectories


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I. INTRODUCTION. Methyl formate, HCOOCH3, is the smallest of esters and a molecule of well known relevance in pure and applied chemistry 1. Recently, it has been detected in astrophysical environments and, since it possesses only two carbon atoms, it is classified as archetypal of the “complex organic molecules” 2-4 of astrochemical and protobiological interest. It is currently being investigated regarding its production and consumption mechanisms, especially because its astrochemical abundance appears anomalously larger than that of its two relevant isomers, acetic acid CH3COOH and glycol aldehyde CH2OH-CHO, in spite of their higher thermodynamic stability 3. Methyl formate, under photolysis, decomposes significantly into methyl alcohol CH3OH and carbon monoxide CO and this decomposition channel is found to prevail in pyrolysis 5. In the present work, we study the photolysis of methyl formate by focusing on the product CO, which, because of its well characterized spectroscopic features, is a favorite probe in the laboratory and in various contexts, including astrophysics 6,7. In our recent studies 8-11 of the photodissociation carried out at various different wavelengths, we determined the energy threshold for the triple fragmentation into H, CH3O and CO, measuring by ion imaging the CO translational energies for sets of selected rotational states of CO in its ground and first excited vibrational levels (v = 0,1), and additionally obtained measures of the H and HCO fragments. The substantial accompanying support of quantum chemical descriptions of the selective rupture of chemical bonds and of molecular dynamics simulations of the evolution of their breakdown along dissociation paths, pointed out the involvement of nonadiabatic effects near a conical intersection in connection with the presence of a signature of roaming, a dissociation mechanism


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characterised by reaction paths accessing high lying regions of the potential energy surface, producing fragments with cold rotational and kinetic energy distributions. Roaming, as an alternative pathway to dissociation products, is an example of a new class of reaction mechanisms 12-14, since it shows a clear experimental signature given by slow and/or cold kinetic and rotational energy distributions of the product fragments. Particularly, slow dissociation fragments have been repeatedly found in photodissociation experiments involving (mainly but by no means exclusively) organic molecules. Roaming has been originally found to occur in the photodissociation of formaldehyde H2CO 15 and later discovered in acetaldehyde CH3CHO16-20, methyl formate 7,8 and propionaldehyde 21, by CO fragments, but this mechanism has been also found in the photodissociation of NO3 27 and of even more complex molecules such as nitrobenzene C6H5NO2 22. In the case of the aldehydes, recent papers 23-25 including extensions to higher members of the series, have shown that, as the molecular size increased, the direct pathway decreased in importance with respect to the roaming one, to the point of becoming negligible (see Ref. 8 for a résumé of some milestones and Ref. 9 for contribution to current debates). Often, the accompanying classical trajectory simulations contributed to attach a meaning to what can be considered as the prevailing significance of the roaming effect, contrasted to either “direct” mechanism or transition state pathways. In order to avoid excessive vagueness and to exclude unwarranted generalizations, two distinct paradigms in our opinion are to be distinguished in applying unambiguously the nomenclature: (i) the simplest, yet perhaps the most genuine picture on origin, nature and manifestation of roaming, is associated to pathways on a single potential energy surface of given elementary


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chemical processes, exhibiting what in older chemical kinetics literature had the characteristic of “loose” transition state, or even of asymptotically flat exit channel, alternative to the “royal road”, namely the minimum energy path (MEP) through the transition state, technically distinguished by the adjective “tight”. (ii) On the other hand, often processes involve more than a single potential surface exhibiting nonadiabatic effects in specific neighborhoods and even at conical intersections. In photochemistry, this was the rule, rather than the exception. In the case of methyl formate HCOOCH3 fragmented by photolysis into CH3OH and CO, the latter serves as the “scout” carrying information from explorations of vast territories of the potential energy surface possibly far away from the minimum energy path. To gain a realistic picture of the roaming/MEP branching of the products in the photodissociation of small and medium-size molecules is a major objective of theoretical and experimental chemistry for all those processes that generate diatomic and triatomic species of interest in modeling of atmospheric chemistry, astrochemistry and combustion. Such information can be inferred from a thorough reconstruction of the energy disposal in the rovibrational degrees of freedom of the product fragments. In this work, quantitative information on the energy disposal into rotations and vibrations of the CO fragments, originated from the photodissociation of methyl formate into CH3OH and CO upon laser irradiation at a wavelength of 248 nm and under effusive beam conditions, is obtained


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by FTIR emission spectroscopy of CO. Vibrational excitation of CO is found up to v = 4, where the corresponding rotational populations are entirely reconstructed and found to be bimodal, due to the existence of two contributions, from MEP and roaming modes respectively. The data presented here establish a net improvement over our previous view of the photodissociation of methyl formate 8-11, which were obtained from experiments conducted under jet-cooled conditions (at the same photolysis wavelength) limited to the detection of CO (v = 0,1) for some selected rotational quantum numbers only, the probing of higher vibrational states being prevented by the poor signal-to-noise ratio of the 2+1 resonance-enhanced multiphoton ionization (REMPI) measurements for CO(v) with v > 1. This study is therefore intended a completion of previous ones 8-11, based on ion imaging.


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II. EXPERIMENTAL AND THEORETICAL METHODS II.1 The experimental method In this integrated experimental and theoretical work, we have characterized the mechanisms of decomposition of the methyl formate, HCOOCH3, into CH3OH and CO, probing the energy disposal in the rovibrational levels of the fragment CO. The experiment was designed to study the photodissociation of the methyl formate by measuring the Fourier-Transform-Infra-Red (FTIR) emission spectra of the fragments CO(v), for different vibrational quantum number v > 0, reconstructing the energy disposal in their degrees of freedom. The recorded spectra enabled us to reconstruct the rotational energy distributions for each specific quantum number and to estimate the relative roaming/MEP contributions to the formation of CO(v ) products. The CO(v ) vibrational population extracted without difficulty from the FTIR spectra, extends to the states v = 1,2,3 and 4, with full rotational bands. Higher vibrational states are observed with less population and unfavorable signal-to-noise ratio. The laser irradiation wavelength was set at 248 nm, to make the photodissociation occur in a sub-threshold regime with respect to triple-fragmentation into H, CO and CH3O which is therefore excluded and does not interfere. As pointed out in a previous work (see Ref. 11), slow CO fragments from photodissociated methyl formate are generated abundantly 11,26 through the H + CO + CH3O channel at irradiation wavelength smaller than 248 nm, causing ambiguities in the CO products attribution to roaming pathways.


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Differently from previous ion-imaging experiments, the IR emission spectra were recorded from samples generated under effusive beam conditions at 300 K, with the addition of Ar in the reaction chamber. The internal conversion (IC) and intersystem crossing (ISC) processes may be enhanced in the effusive regime, benefited by the increased rate of level-to-level coupling between electronic states. Under such circumstances, the signal-to-noise ratio increased sufficiently to allow for a detailed analysis of the CO spectra even in case of relatively small Einstein emission coefficients in the IR region. Furthermore, because the FTIR experiments were carried out at 300 K and in presence of Ar, the probability of cluster formation was strongly reduced, contrary to the previous imaging beam experiments that had to deal with the clustering phenomena, typical of jet-cooled conditions. The reconstruction of the full rotational bands of the CO(v) fragments for v = 1, 2, 3, 4, and the roaming/MEP relative contributions to the CO formation have been carried out by the procedure described in next sections. II.2 Reconstruction of the rovibrational spectrum After each spectrum acquisition, the knowledge of the infrared emission frequency and the Einstein spontaneous emission coefficient Av,J corresponding to each rovibrational line, is needed to reconstruct the rovibrational population of the CO fragments with the aid of spectral simulations. The procedure to determine the emission frequencies and the Einstein spontaneous emission coefficients is as follows: knowing the CO vibrational frequency and rotational constants27,28, the rovibrational emission frequencies can be calculated via an analytical expression of the closed-shell diatomic rovibrational energy (see section 7.1 of Ref. 29). With the


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potential energy curve and electric dipole moment function both adopted from Ref.

30,

the

Einstein spontaneous emission coefficients for each rovibrational line can be expressed in term of the square moduli of the dipole moment matrix elements (see section 7.6 and 9.5 of Ref. 29). To calculate the dipole moment matrix elements of the vibrational wavefunction, one first numerically computes the vibrational wavefunction by solving the nuclear Schrödinger equation for the CO diatomic molecule via a discrete variable representation (DVR) approach 31. Then, the off-diagonal dipole moment matrix elements are computed explicitly by using the obtained vibrational wavefunctions. In the subsequent data analysis aimed at revealing bimodal distributions, each rotational population is assumed, in principle, to be characterized by a Boltzmann distribution law (see next Section). In order to minimize the uncertainties due to possibly low signal-to-noise ratios for the CO spectral acquisitions at different delay times, time-dependent plots of the rotational temperatures serve to reduce the uncertainty propagated in the extrapolation process. Indeed, for a linear plot of the rotational component versus the delay time, the propagated uncertainty is proportional to the reciprocal of the slope; e.g. the uncertainty is reduced by a factor of two, if the slope is two. In this way, the uncertainty can be further reduced by increasing the number of data points in the plot. Accordingly, the time-resolved FTIR method, within the 5-μs response time of our apparatus, is sensitive enough to distinguish different rotational population components with the aid of


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spectral simulations. The time-dependent ratio of low-/high-J population component is then extrapolated to a real zero time at which the collisional effect by Ar is removed. For instance, the same method yielded a bimodal rotational distribution of CO(v = 1) from acetaldehyde photodissociated at 308 nm 23 and also propionaldehyde and isobutyraldehyde 32,33. In the photodissociation of acetaldehyde at 308 nm, the obtained low-/high-J ratio of (10±2)/ (90±5) at v = 1 is consistent with the value of 13/87 for CO at v = 1 obtained by Houston and Kable by the laser-induced fluorescence method

34.

Such consistency of different methods is

indicative of the reliability of our approach to obtain rotational populations of CO for vibrational quantum numbers up to 4.

II.3 Bimodality of the rovibrational distributions By assuming Gaussian line shapes, the rovibrational spectral line, as a function of the wavenumber, is represented as Iv,J (˜ ⌫ ) = Pv,J Av,J f (˜ ⌫)

(1)

⌫ ) the Gaussian line shape where A v,J is the Einstein spontaneous emission coefficient, f (˜

function with area normalized to unity and Pv,J the population of the given rovibrational state, where the subscripts v and J denote the initial vibrational and rotational quantum numbers of the emission process. In the case of a bimodal distribution, the population Pv,J is expressed in the following parametric form:


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

Pv,J = Nv Cv

(2J+1)exp( EJ /KB Tv1 ) Q v1

+ (1

Cv )

(2J+1)exp( EJ /KB Tv2 ) Q v2

(2)

where Tv1 and Tv2 are rotational temperatures, Qv1 and Qv2 are the rotational partition functions of the corresponding Boltzmann distributions, Cv is a coefficient ranging between 0 and 1 and Nv is the population of the given vibrational state, as obtained directly from the experiment. For

any given vibrational state, the three parameters, Cv, Tv1 and Tv2, are adjusted to reconstruct the observed bimodal Boltzmann rotational distribution. According to the procedure described above and in Section II.2, a MATLAB language program has been built up to perform the population calculations and the spectral simulations.

II.4 Potential energy surface and trajectory calculations In order to run quasiclassical trajectory calculations (QCT) simulating the photodissociation process, a potential energy surface of the methyl formate (HCOOCH3), in the electronic ground state, has been constructed. Due to the relatively high number of atoms involved and since our experiment is designed to probe the rovibrational energies of CO 8,11, the intramolecular interaction of the methyl formate has been reduced to that of a four-body system, by considering the methoxy group, CH3O as frozen in its equilibrium geometry, here denoted as “M” and representing the molecule simply as HCOM. In Figure 1 the four-body effective model is shown along with the distances and angular coordinates involved in the dissociation into HM(= H-OCH3) and CO. The potential energy


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surface of this four-center model system has been generated in a parametric form following a many-body expansion approach 35, earlier successfully applied to few-body systems including formaldehyde 36,37, whose structure HCOH resembles that of our four-body model of methyl formate. Accordingly, the potential energy of the effective four-body system, VHCOM , can be expressed by the following expansion: (2)

(2)

(2)

(2)

(2)

(2)

(3)

(3)

VHCOM = VCH + VCO + VOH + VOM + VHM + VCM + VHCO + VM CO + (3) (4) VHCM + VHCOM

where the superscripts in parenthesis (n) refer to n-body interaction terms and the subscripts indicate the interacting atoms, or atom group in the case of M. The interaction energy in the (1)

(1)

(1)

(1)

asymptotic limit of the four single-body terms, VH , VC , VO and VM , not appearing in the above equation, is set at zero. The classical trajectory simulations of the methyl formate photodissociation have been run assigning to the vibrational degrees of freedom an energy of 115.29 kcal/mol, corresponding to the experimental condition of irradiating the sample at 248 nm. A zero-point energy of 17.97 kcal/mol was considered (then added to the total energy), according to the harmonic vibrational frequencies of our model potential energy surface. In the simulations, the initial distribution of energy among the molecular degrees of freedom should not be uniform. In particular, having failed, in trial calculations, to find slow products originating from large samplings of trajectories evolving along the minimum energy path through the transition state saddle, we systematically studied the effect of trajectories generating initial geometries by a random sampling of the configuration space in the neighbourhood of the tip of the lower cone of a conical intersection. In


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fact, available quantum mechanical and molecular dynamics investigations have suggested the role of conical intersections11,38,39 in favoring an alternative pathway to the CH3OH + CO channel for the system by selectively funneling energy into specific degrees of freedom. This pathway involves configurations highly distorted with respect to those occurring near to the ground state and to the lower transition state, being in particular elongated in the direction of the C-O bond connecting the incipiently separating HCO and CH3O radicals 25,26 (see Fig. 1). The consequences of this alternative route to products shows up by the presence of a significant fraction of CO fragments with low kinetic and rotational energy. The main feature of the selected initial geometries is the HOC--OCH3 bond rather elongated with respect to the equilibrium length. These initial configurations are arguably those populated by the conical intersection via which the internal conversion takes place from the S1 to the S0 adiabatic surface. Then we have been looking for possible branching ratios of exit channels, independently of the nonadiabatic mechanism due to the coupling responsible for the transition from the upper S1 surface (see Fig. 1). Note that starting trajectories with this geometry of the molecule has the meaning of locally exciting the OC--OCH3 bond, rising the energy of the molecule up to nearly the radical channel HCO + CH3O (see Fig. 1). The difference between the energy of the starting elongated configurations (each time different due to the slightly different geometry) and the total value of ~115 kcal/mol, was randomly assigned (with the addition of a contribution due to the zero-point energy) to the remaining degrees of freedom, in generating the trajectory initial velocities. A detailed and extensive account, both of the construction of the surface and of the simulations is given in Refs. 8 and 19.


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III. RESULTS AND DISCUSSION The interval between the estimated radical channel energy of ~ 97 kcal/mol 10,11,38,39 (see Fig. 1) and the opening of the triple fragmentation pathway (above irradiation energy of ~ 115 kcal/mol equivalent to 248 nm 11,39) is the sub-threshold energy strip where the CO originating from the roaming channel can be observed separately with no contribution due to the occurrence of triple fragmentation (see Fig. 1 and previous section). For the roaming mechanism to be likely, it is required that methyl formate following excitation, and prior to dissociation, behaves as a weakly bound HCO—OCH3 complex, at the entrance of the radical channel HCO + CH3O. Accordingly, the information on the existence of a conical intersection in the energy range of the experiments, available from theoretical calculations 38, provided support to the idea of a mechanism for the photoexcitation whose first step is the excitation of the system to an upper branch of the surface, designated S1, followed by the delivery back to the lowest energy surface S0, taking place in a region of the configuration space corresponding to the tip of the above mentioned conical intersection and far away from the region of the minimum energy structure. It must be expected (it can be theoretically predicted 38,39) and was confirmed by previous experiments 8,11, that a certain amount of radical products HCO + CH3O can be observed, besides the CO + CH3OH products, and that only two dissociation channels are effective. HCO could dissociate into H + CO, but again it must be remarked that triple dissociation, resulting from further breaking of HCO into H + CO, does not occur under the experimental conditions of irradiating at 248 nm 11,26,

in such a way that all the observed CO comes from the same dissociation channel.

Fig. 2 shows raw data points that were obtained by our experiment in the form of Boltzmann plots of rotational populations for CO(v) for v = 1 and 2. A nonlinear fitting procedure of the the


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state populations Pv,J obtained from Eq. (1) by means of spectral simulations, and based on Eq. (2), allowed us to obtain the rotational temperatures T< and T> of the cold and hot rotational bands and their relative populations, corresponding to the populations of CO fragments produced from roaming and MEP pathways, respectively. The existence of two slopes in the logarithmic plot, at low and higher J values, is clearly visible, indicating bimodality due to two active reaction pathways for the CO formation. The rovibrational state populations of CO obtained for the higher vibrational states v =3 and 4 were instead fitted by a single temperature Boltzmann distribution due to a less favourable signal-to-noise ratio. The complete results are shown in Fig. 3, where the steps leading of the reconstruction of the populations for each rotational line from the time-resolved spectra and the spectral simulations to make assignments and obtain the individual rovibrational state populations are shown. The rotational temperatures characteristic of the distributions of the CO(v) fragments in each of the four vibrational levels, have been determined by extrapolation at zero delay time (see previous Section), as shown in the right lower panel of Fig. 3: consistently, the higher vibrational levels exhibit colder rotational energy distributions. The rotational reconstruction is exemplified in Fig. 4. The figure emphasizes the signature of two alternative pathways to CO products, resulting in the bimodal character of the rotational energy disposal of the CO fragments in the v = 1 and v = 2 states (for which the signal-to-noise ratio is favorable). The v = 3 and v = 4 CO vibrational states are less populated and fitted satisfactorily with a single distribution (but it will be shown in the following that simulations predict bimodality for these states also). Higher vibration states


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were also observed in the experiment, but are not reported here due to the very poor signal-tonoise ratio. The use of Boltzmann-type distribution functions in the deconvolution appears justified by the results and permits to extract effective rotational temperatures (see Fig. 4 and Table 1). The roaming/MEP branching ratio can be determined assuming it proportional to the ratio of the cold and hot rotational populations, as obtained by the deconvolution, for each vibrational quantum number. The values normalized are represented in Fig. 5 along with the analogous results obtained from extensive molecular dynamics simulations. The complete set of rotational temperatures and roaming/MEP fractions is summarized in Table 1. The extraction of effective temperatures permits one to estimate the mean energy deposited in each mode, also reported in Table 1. The vibrational population of the CO products is reported in Fig. 5 (right panel) as obtained from experiments and simulations along with a Boltzmann distribution at T = 10000 K. In order to avoid arbitrary biases, no binning into the CO quantum vibrational level spacings has been performed, but a finer one to obtain a classical-like continuous vibrational energy distribution. For the v = 1 and v = 2 vibrational states, the experimental values of the population fraction of CO generated from the two alternative roaming and MEP pathways are shown and compared with the corresponding results from trajectory calculations. The comparison shows that the vibrational populations of CO, a sum of the roaming and MEP fractions (only theoretical for v = 3 and v = 4) are in good agreement between theory and experiment. In the left panel, the complete classical vibrational energy distribution obtained from the trajectory calculations is shown to follow closely the Boltzmann distribution at T = 10000 K, within uncertainty of ~10


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%. It is worth noting that estimating the areas in the left panel of Fig. 5, ~ 80% of the CO vibrational energy is included in the experimentally observed vibrational population (up to v = 4). The branching to each pathway can be obtained by summing over the full distribution of the rotational levels. This is indeed an advantage of the FTIR emission spectroscopy technique.In contrast, the ion imaging technique, in provision of the translational energy characterization, permits to obtain results at only a few selected rotational levels for v = 0 and gives very limited information for v = 1 8-11. As a result of the present work, the pattern emerges of how energy globally goes into rotational and vibrational modes. Table 1 can be consulted for the quantitative assessment of these findings on the energy disposal.

IV. SUMMARY AND CONCLUSIONS In this integrated experimental and theoretical investigation of methyl formate photolysis, a wavelength of 248 nm has been used to avoid the opening of triple fragmentation into H, CH3O and CO. Time-resolved Fourier-Transform InfraRed (FTIR) emission spectroscopy technique has been employed to acquire the rovibrational spectra of CO fragment. The molecules were seeded in a room temperature (300 K) Ar expansion, helping to enhance the collision-induced internal conversion that populates the CO vibrational states up to v = 4, and allowing for the investigation of the roaming branching behavior as a function of the vibrational state. The rotational population distribution was found to be bimodal for v = 1 and 2, the cold rotational component was ascribed to the roaming mechanism and the hot one ascribed to a direct molecular mechanism, following a minimum energy path involving the transition state. Bimodality could not be disentangled for the v = 3 and 4 populations measured with a lower signal-to-noise ratio.


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The quasi-classical trajectory calculations were performed on a reduced dimensional potential energy surface. A careful sampling of the trajectory initial configurations in the neighborhood of the conical intersection between the excited singlet S1 and ground state S0 , yielding molecular geometries characterized by an elongation of the HCO—OCH3 bond, allowed to take into account for the role of nonadiabatic transitions. The trajectory evolution produced the CO rotational and vibrational energy distributions in quantitative consistency with the experimental findings, and provided new insight into the occurrence of roaming versus the direct dissociation pathway involving a transition state, in the methyl formate. The conclusions that we can draw from our experimental results, demonstrate quantitatively that when molecules are highly excited, but below the threshold for triple dissociation, the energy deposited in the CO fragment is essentially found in vibrations, while rotation modes are colder. Roaming persists as a substantial phenomenon also for the production of high vibrational CO states.


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Figure 1. A picture of the photodissociation of HCOOCH3 along the molecular elimination channel to CO + CH3OH and the radical channel to HCO + CH3O. The molecules, excited from the ground S0 to the S1 surface, populate back S0 through non-adiabatic crossings in the neighborhood of the tip of the conical intersection. This is a rotated view, with respect to the previous paper 11, where details are given about inherent uncertainties on the energies. Here we highlight the radical channel and the vibrational excited levels of the CO product which are probed by FTIR emission spectroscopy just below the opening of the triple fragmentation channel. There CO can be produced by molecules wandering in the radical channel before falling down in the molecular exit valley.


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Figure 2. Boltzmann plots of raw data of rotational populations for v = 1 and v =2. Bimodality is exhibited by changes in slopes (dashed lines are a guide for readers’ eyes).


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experimental spectrum

reconstructed spectrum

Figure 3. Rovibrational spectra at different time delays of CO dissociated from methyl formate at 248 nm obtained by time-resolved FTIR emission spectroscopy (left side). A spectral reconstruction (red) of the spectrum (black) at 7.5 μs is shown with the corresponding assignments of the rotovibrational levels within the P and R branches. Extrapolation at zero delay time is shown for CO (v = 1,2,3 and 4) giving the state-specific rotational temperatures after the reconstruction outlined according to Fig. 4.


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Figure 4. The reconstructed rotational quantum number distributions from the estimated rotational temperature values from Boltzmann deconvolutions. Two components, designated by T< and T> (see also Table 1) are disentangled and attributed as arising from events emerging

directly from minimum energy path into the molecular channel CH3OH + CO, or from roamingtype paths along the radical channel.


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Figure 5. Left panel: vibrational energy distribution as obtained from classical trajectory calculations, indicating a Boltzmann behavior of the CO vibrational energy disposal. The right panel shows, on an enlarged abscissa, the experimental (black circles) and theoretical (blue squares) roaming, MEP and total CO(v) populations for v = 1,2,3 and 4, normalized with respect to the Boltzmann distribution at 10000 K as a fitting curve. Here the binning of energies as v ± 1/2 boxes permits to extract quantum CO vibrational populations; dashed curves are aids to the eye of the reader.


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TABLES Table 1. Experimental rotational temperatures and populations for the CO vibrational levels.

CO(v )

v=1

v=2

v=3

v=4

Rotational temperature (K)

T> = 1080 ± 70 T< = 470 ± 50

T> = 920 ± 50 T< = 430 ± 50

T = 800 ± 50

T = 730 ± 40

Energies (kcal/mol)

> Erot = 2.15 ± 0.04 < Erot = 0.93 ± 0.03

> Erot = 1.83 ± 0.03 < Erot = 0.85 ± 0.03

Erot = 1.59 ± 0.03

Erot = 1.45 ± 0.02

Normalized populationa

M EP : 0.46 ± 0.07 roam : 0.20 ± 0.03 total : 0.66 ± 0.09

M EP : 0.39 ± 0.08 roam : 0.09 ± 0.03 total : 0.48 ± 0.09

total : 0.34 ± 0.04

total : 0.25 ± 0.02

a setting

the population at v = 0 as equal to that of the zero-point energy of the classical distribution (see Fig. 5).

AUTHOR INFORMATION Corresponding Authors *Email: [email protected] **Email: [email protected] ACKNOWLEDGMENT The Ministry of Science and Technology, Taiwan, Republic of China, supported this work under contract no. NSC 102-2113-M-002-009-MY3 and the stay of T. Kasai at the Department of Chemistry, National Taiwan University. A. Lombardi acknowledges financial support from MIUR PRIN 2010-2011 (contract 2010ERFKXL 002) and EGI Inspire. V. A. thanks Brazilian CAPES for grant as Distinguished Visiting Professor at Universidade da Bahia. Thanks are also due to IGI (Italian Grid Infrastructure) and COMPCHEM Virtual Organization for the allocated


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