Coauthored Publications with: Nakamura

Conference Paper

Lin, K-C, Nakamura M, Yang, S. J, Kasai T, Che D-C, Lombardi A, Palazzetti F, Aquilanti V.  2017.  Angular distribution of bromine atomic photofragment in oriented 2-bromobutane via hexapole state selector. AIP Conference Proceedings. 1906 Abstract

In this work, an asymmetric top molecule 2-bromobutane has been successfully oriented by using hexapole state selector combined with orientation field, followed by detection of the bromine atomic photofragment distribution in the photolysis. The photofragment is produced in both the ground Br (2P3/2) and the excited Br (2P1/2) electronic states and both channels are studied by the slice imaging technique, revealing new features in the stereodynamic vectorial properties with respect to previous investigations on non-oriented molecules. © 2017 Author(s).
Palazzetti, F, Lombardi A, Nakamura M, Yang S-J, Kasai T, Lin K-C, Tsai P-Y, Che D-C.  2016.  Rotational state-selection and alignment of chiral molecules by electrostatic hexapoles. AIP Conference Proceedings. 1790 Abstract

Electrostatic hexapoles are revealed as a powerful tool in the rotational state-selection and alignment of molecules to be utilized in beam experiments on collisional and photoinitiated processes. In the paper, we report results on the application of the hexapolar technique on the recently studied chiral molecules propylene oxide, 2-butanol and 2-bromobutane, to be investigated in selective photodissociation and enantiomeric discrimination. © 2016 Author(s).
Palazzetti, F, Lombardi A, Yang S-J, Nakamura M, Kasai T, Lin K-C, Che D-C, Tsai P-Y.  2016.  Stereodirectional photodynamics: Experimental and theoretical perspectives. AIP Conference Proceedings. 1790 Abstract

Hexapole oriented 2-bromobutane is photodissociated and detected by a slice-ion-imaging technique at 234 nm. The laser wavelength corresponds to the C - Br bond breaking with emission of a Br atom fragment in two accessible fine-structure states: the ground state Br (2P3/2) and the excited state Br (2P1/2), both observable separately by resonance-enhanced multiphoton ionization (REMPI). Orientation is evaluated by time-of-flight measurements combined with slice-ion-imaging. © 2016 Author(s).

Journal Article

Kasai, T, Che D-C, Tsai P-Y, Nakamura M, Muthiah B, Lin K-C.  2018.  Roaming and chaotic behaviors in collisional and photo-initiated molecular-beam reactions: a role of classical vs. quantum nonadiabatic dynamics. Rendiconti Lincei. 29:219-232., Number 2 AbstractWebsite

A new reaction scheme is proposed to account for roaming and chaotic behaviors in collisional and photo-initiated molecular-beam reactions, where nonadiabatic dynamics plays a key role and the collapse of superposition of wave functions is considered to be important in the beginning of the present scheme. Since the feature of molecular orbitals of reagents is crucial in reaction, we showed how to map out the spatial distribution of the relevant HOMO molecular orbitals of CH3Cl in the impact of fast electrons. We identified by experiment that the multiple overlap of nearby molecular orbitals affects even the vibrational motion of adjacent molecule DCl of the transient [ClDCl] chemical species. We also showed dynamical steric effects in the HBr + OH four-atom reaction as a manifestation of the nonadiabatic dynamics in complex systems. The roaming mechanism in the photo-initiated reaction of methyl formate is clarified in detail by experiment as well as the QCT trajectory calculation, where the conical intersection region plays an essential role. We suggest that two types of roaming trajectories coexist, i.e., deterministic and chaotic roaming trajectories based on classical trajectory calculations. To clarify the nonadiabatic dynamics in the roaming mechanism for non-collinear three-dimensional (3D) collisions, a new model of the 3D Polanyi rule is proposed as the extension of the well-established 2D Polanyi rule. In the 3D Polanyi rule, it is expected that the curvature and torsion of Frenet–Serret formulas in three-dimensional space would provide us key concepts in understanding reaction dynamics. © 2018, Accademia Nazionale dei Lincei.
Nakamura, M, Yang S-J, Tsai P-Y, Kasai T, Lin K-C, Che D-C, Lombardi A, Palazzetti F, Aquilanti V.  2016.  Hexapole-Oriented Asymmetric-Top Molecules and Their Stereodirectional Photodissociation Dynamics. Journal of Physical Chemistry A. 120:5389-5398., Number 27 AbstractWebsite

Molecular orientation is a fundamental requisite in the study of stereodirected dynamics of collisional and photoinitiated processes. In this past decade, variable hexapolar electric filters have been developed and employed for the rotational-state selection and the alignment of molecules of increasing complexity, for which the main difficulties are their mass, their low symmetry, and the very dense rotational manifold. In this work, for the first time, a complex molecule such as 2-bromobutane, an asymmetric top containing a heavy atom (the bromine), was successfully oriented by a weak homogeneous field placed downstream from the hexapolar filter. Efficiency of the orientation was characterized experimentally, by combining time-of-flight measurements and a slice-ion-imaging detection technique. The application is described to the photodissociation dynamics of the oriented 2-bromobutane, which was carried out at a laser wavelength of 234 nm, corresponding to the breaking of the C-Br bond. The Br photofragment is produced in both the ground Br (2P3/2) and the excited Br (2P1/2) electronic states, and both channels are studied by the slice imaging technique, revealing new features in the velocity and angular distributions with respect to previous investigations on nonoriented molecules. © 2016 American Chemical Society.
Tsai, P-Y, Che D-C, Nakamura M, Lin K-C, Kasai T.  2010.  Orientation dependence in the four-atom reaction of OH + HBr using the single-state oriented OH radical beam. Physical Chemistry Chemical Physics. 12:2532-2534., Number 11 AbstractWebsite

The orientation dependence for the Br atom formation in the reaction of the oriented OH radicals with HBr molecules at 0.26 eV collision energy has been observed for the first time using the hexapole electric field, and we found that the reaction cross-section for O-end attack is more favorable than that for H-end attack by a factor of 3.4 ± 2.3. © the Owner Societies.
Tsai, P-Y, Che D-C, Nakamura M, Lin K-C, Kasai T.  2011.  Orientation dependence for Br formation in the reaction of oriented OH radical with HBr molecule. Physical Chemistry Chemical Physics. 13:1419-1423., Number 4 AbstractWebsite

The orientation dependence of Br-atom formation in the reaction of the oriented OH radical with the HBr molecule using the hexapole electrostatic field was studied. Experimental results for the orientation dependence in the reaction were analyzed using a Legendre polynomial fit. The results show two reactive sites. It was found that O-end attack is most favored for this reaction, and that H-end attack also shows a pronounced reactivity. The reactivity of the side-ways attack was found to be small. By comparing the results of the orientation dependence in the reaction with studies of inelastic collisions and theoretical calculations, two reaction pathways are proposed. Reaction by O-end attack is followed by a direct abstraction of the H-atom from the HBr molecule. The mechanism for H-end attack may have H-atom migration from HBr to form the water molecule. © 2011 the Owner Societies.
Nakamura, M, Yang, S. J, Lin K-C, Kasai T, Che D-C, Lombardi A, Palazzetti F, Aquilanti V.  2017.  Stereodirectional images of molecules oriented by a variable-voltage hexapolar field: Fragmentation channels of 2-bromobutane electronically excited at two photolysis wavelengths. Journal of Chemical Physics. 147, Number 1 AbstractWebsite

The asymmetric-top molecule 2-bromobutane is oriented by means of a hexapole state selector; the angular distribution of the bromine atom photofragment, for the two fine-structure components, is acquired by velocity-map ion imaging. The molecular beam, spatially oriented along the time-of-flight axis, is intersected with a linearly polarized laser, whose polarization is tilted by 45° with respect to the detector surface. To obtain the mixing ratio of the perpendicular and parallel transitions, the fragment ion images and angular distributions can be appropriately simulated to give insight on the population mechanism of the specific electronic state involved at each selected excitation wavelength. The photofragment images obtained at 238.6 nm yielded an asymmetry factor β1 of 0.67, indicative of the extent of molecular orientation, and an anisotropy parameter β2 of 1.03, which is a signature of a prevailing parallel transition along the C-Br axis. When the photolysis wavelength is tuned to 254.1 nm, the corresponding angular distribution is less asymmetric (β1 = 0.24) and the obtained small value β2 = 0.12 is a characteristic of a predominantly perpendicular transition. The photofragment angular distributions are also affected by hexapole voltage, especially regarding the asymmetry factor, and this aspect provides information on the effect of molecular orientation. © 2017 Author(s).
Nakamura, M, Che D-C, Tsai P-Y, Lin K-C, Kasai T.  2013.  Alignment selection of the metastable CO(a 3π1) molecule and the steric effect in the aligned CO(a 3π1) + NO collision. Journal of Physical Chemistry A. 117:8157-8162., Number 34 AbstractWebsite

The aligned metastable CO(a 3π1) molecular beam was generated by an electronic excitation through the Cameron band (CO a 3Π1 ← X 1Σ+) transition. Beam characterization of the aligned molecular beam of CO(a 3Π1) was carried out by (1 + 1) REMPI detection via the b 3Σ+ state. The REMPI signals showed the clear dependence on the polarization of the pump laser, and the experimental result was well reproduced by the theoretical simulation. This agreement confirms that aligned metastable CO(a 3Π1) can be generated and controlled by rotating polarization of the pump laser. By using this technique, a single quantum state of CO(a 3Π1) can be selected as a metastable molecular beam. The steric effect in the energy-transfer collision of CO(a 3Π1) with NO forming the excited NO was carried out with this aligned CO(a 3Π1) molecular beam. We find that the sideways orientation of CO(a 3Π1) is more favorable in the formation of the excited NO(A 2Σ+, B 2Π) than that for the axial collisions. The obtained steric effect was discussed with the aid of the spatial distribution of CO(a 3Π1) molecular orbitals, and we find that specific rotational motion of CO(a 3Π1) in each state may not be a dominant factor in this energy-transfer collision. © 2013 American Chemical Society.
Palazzetti, F, Tsai P-Y, Lombardi A, Nakamura M, Che D-C, Kasai T, Lin K-C, Aquilanti V.  2013.  Aligned molecules: Chirality discrimination in photodissociation and in molecular dynamics. Rendiconti Lincei. 24:299-308., Number 3 AbstractWebsite

Emergence of biochemical homochirality is an intriguing topic, and none of the proposed scenarios has encountered a unanimous consensus. Candidates for naturally occurring processes, which may originate chiral selection, involve interaction of matter with light and molecular collisions. We performed and report here: (1) simulations of photodissociation of an oriented chiral molecule by linearly polarized (achiral) light observing that the angular distribution of the photofragments is characteristic of each enantiomer and both differ from the racemic mixture; and (2) molecular dynamics simulations (elastic collisions of oriented hydrogen peroxide, one of the most simple chiral molecules, with Ne atom) demonstrating that the scattering and the recoil angles are specific of the enantiomeric form. The efficacy of non-chiral light (in the case of photodissociation) and of non-chiral projectile (in the case of collisions) is due to the molecular orientation, as an essential requirement to observe chiral effects. The results of the simulations, that we report in this article, provide the background for the perspective realization of experiments which go beyond the well-documented ones involving interaction of circularly polarized laser (chiral light) with the matter, specifically by making use of non-chiral, i.e. linearly polarized or unpolarized light sources, and also by obtaining chiral effects with no use at all of light, but simply inducing them by molecular collisions. The case of vortices is discussed in a companion paper. © 2013 Accademia Nazionale dei Lincei.
Nakamura, M, Chang H-P, Lin K-C, Kasai T, Che D-C, Palazzetti F, Aquilanti V.  2019.  Stereodynamic Imaging of Bromine Atomic Photofragments Eliminated from 1-Bromo-2-methylbutane Oriented via Hexapole State Selector. Journal of Physical Chemistry A. 123(31):6799-6811. AbstractWebsite

Both single-laser and two-laser experiments were conducted to look into the ion-imaging of Br*(2P1/2) and Br(2P3/2) photofragmented from 1-bromo-2-methylbutane in the range 232-240 nm via a detection scheme of (2+1) resonance-enhanced multiphoton ionization. The angular analysis of these photofragment distributions yields the anisotropy parameter β = 1.88 ± 0.06 for the Br∗ excited state which arises from a parallel transition, while β = 0.63 ± 0.09 for the Br ground state indicates the contribution from both a perpendicular transition and a non-adiabatic transition. When a hexapole coupled with an orienting field was implemented, the parent molecules are spatially oriented to yield an orientation efficiency |«cos θ »| of 0.15. Besides the χ angle between the recoil velocity v and the transition dipole moment μ, orienting molecules allows for the evaluation of the angle α between v and the permanent molecular dipole moment d. The angular analysis of Br∗ photofragment distribution yields χ = 11.5° and α in the range from 160° to 180° with weak dependency. In the two-laser experiments, the angular anisotropy of Br photofragment distribution was found to be smaller (0.38 ± 0.10) when the photolysis wavelength was red-shifted to 240 nm, suggesting the increasing contributions from perpendicular transitions. © 2019 American Chemical Society.
Lin, K-C, Tsai P-Y, Chao M-H, Nakamura M, Kasai T, Lombardi A, Palazzetti F, Aquilanti V.  2018.  Roaming signature in photodissociation of carbonyl compounds. International Reviews in Physical Chemistry. 37:217-258., Number 2 AbstractWebsite

An alternative to the transition state (TS) pathway, the roaming route, which bypasses the minimum energy path but produces the same molecular products, was recently found in photodissociation dynamics. This account describes signatures of roaming in photodissociation of the carbonyl compounds, specifically methyl formate and aliphatic aldehydes. Methyl formate was promoted to the excited state, followed by internal conversion via a conical intersection. Then, the energetic precursor dissociated to fragments which proceeded along either TS or roaming path. In contrast to the lack of a roaming saddle point found in methyl formate, the structure of the roaming saddle point for each of a series of aliphatic aldehydes comprises two moieties that are weakly bound at a distance. As its size increases, the energy difference between the TS barrier and the roaming saddle point increases and the roaming pathway becomes increasingly dominant. Experimentally, the rotational-level dependence of the roaming route was measured with ion imaging, while the vibrational-state dependence was observed with time-resolved Fourier-transform infrared emission spectroscopy. The roaming signature was verified theoretically by quasi-classical trajectory (QCT) calculations. As an alternative to the QCT method, a multi-center impulsive model was developed to simulate the roaming scalar and vector properties. © 2018 Informa UK Limited, trading as Taylor & Francis Group.
Nakamura, M, Tsai P-Y, Kasai T, Lin K-C, Palazzetti F, Lombardi A, Aquilanti V.  2015.  Dynamical, spectroscopic and computational imaging of bond breaking in photodissociation: Roaming and role of conical intersections. Faraday Discussions. 177:77-98. AbstractWebsite

Recent experimental and theoretical advances in the study of the dissociation of excited molecules are revealing unexpected mechanisms, when their outcomes are tackled by combining (i) space-time ion imaging of translational features, with (ii) spectroscopic probing of rotational and vibrational distributions; crucial is the assistance of (iii) the quantum chemistry of structural investigations of rearrangements of chemical bonds, and of (iv) the simulations of molecular dynamics to follow the evolution of selective bond stretching and breaking. Here we present results of such an integrated approach to methyl formate, HCOOCH3, the simplest of esters; the main focus is on the rotovibrationally excited CO (v = 1) product and in general on the energy distribution in the fragments. Previous laser studies of dissociation into CO and CH3OH at a sequence of various wavelengths discovered signatures of a roaming mechanism by the late arrival of CO (v = 0) products in time-of-flight ion imaging. Subsequent detailed investigations as a function of excitation energy provided the assessment of the threshold, which opens for triple breakdown into CO and further fragments H and CH3O, as spectroscopically characterized by ion imaging and FTIR respectively. Accompanying quantum mechanical electronic structure calculations and classical molecular dynamics simulations clarify the origin of these fragments through "roaming" pathways involving incipient radical intermediates at energies below the triple fragmentation threshold: a specific role is played by nonadiabatic transitions at a conical intersection between ground and excited states; alternative pathways focalize our attention to regions of the potential energy surfaces other than those in the neighbourhoods of saddle points along minimum energy paths: eventually this leads us to look for avenues in reaction kinetics beyond those of venerable transition state theories. This journal is © The Royal Society of Chemistry.
Nakamura, M, Palazzetti F, Tsai P-Y, Lin K-C, Kasai T, Che D-C, Lombardi A, Aquilanti V.  2019.  Vectorial imaging of the photodissociation of 2-bromobutane oriented: Via hexapolar state selection. Physical Chemistry Chemical Physics. 21(26):14164-14172. AbstractWebsite

Molecular orientation techniques are becoming available in the study of elementary chemical processes, in order to highlight those structural and dynamical properties that would be concealed by random rotational motions. Recently successful orientation was achieved for asymmetric-top and chiral molecules of much larger complexity than hitherto. In this work, we report and discuss the correlation between the vectors' photofragment recoil velocity v, transition dipole moment μ, and permanent dipole moment d in a dissociation experiment on hexapole oriented 2-bromobutane, photoinitiated by a linearly polarized laser. The sliced ion images of the Br∗(2P1/2) and Br(2P3/2) photofragments were acquired at 234.0 and 254.1 nm, respectively, by a (2 + 1) resonance-enhanced multiphoton ionization technique. A detailed analysis of the sliced ion images obtained at a tilting angle 45° of laser polarization provides information on the correlation of the three vectors, which are confined by two polar angles α and χ and one azimuthal angle φμd in the recoil frame. The sliced ion images of Br fragments eliminated individually from the enantiomers at 254.1 nm yield an asymmetric factor close to zero; for this reason the photofragment angular distributions do not show significant differences. The elimination of the Br∗ fragment at 234.0 nm is mainly correlated with a parallel transition, giving rise to a large anisotropy parameter of 1.85, and thus can be considered as a single state excitation. The resulting recoil frame angles are optimized to 163° ± 8° and 164° ± 1° for α and χ, respectively, whereas φμd is approaching 0° for the best fit. Since for the present molecule, the three vectors have an only slight spatial arrangement, the photofragment angular distributions of the two enantiomers do not show appreciable differences. Theoretical and computational simulations provide us the basis to state that oriented enantiomers can be discriminated on-the-fly in photodissociation processes even initiated by non-circularly polarized light, provided that the three vectors encountered above have specific three-dimensional arrangements. The fact that Br fragment elimination involves a multi-potential dissociation carries uncertainties in theoretical estimates of the vector direction. Therefore, this work represents a preliminary but significant step on the road to chiral discrimination on-the-fly, which is shown to be best propitiated in molecules where vectors are far from having degenerate mutual angular directions. © 2019 the Owner Societies.