Papers by behnam nikoobakht
JOHN STANTON SPECIAL ISSUE: THEORY MEETS EXPERIMENT UV absorption spectrum and photodissociation dynamics of CH2OO following excitation to the B1A′ state
The spectroscopy and dynamics of the smallest Criegee intermediate CH2OO following UV excitation ... more The spectroscopy and dynamics of the smallest Criegee intermediate CH2OO following UV excitation to the B state is studied theoretically, based on multireference electronic wave functions and a quantum dynamical approach for the nuclear motion. Two interacting electronic states and two nuclear degrees of freedomare considered in the dynamical treatment. TheUV absorption spectrum is found to agree very well with available experimental recordings when accounting for broadening effects due to vibrational and rotational congestion and lifetime effects. Upon higher resolution two different energetic regimes can be approximately distinguished, a higher-energy regime where the spectral envelope is broad and structureless, and dissociation is prompt, and a lower-energy regime featuring narrow resonances which are supported by the shallow well occurring at intermediate O–O distances, and decay by tunnelling to the repulsive outer part of the potential energy surface. The importance of nonadi...
UV absorption spectrum and photodissociation dynamics of CH2OO following excitation to the B1 A′ state
Molecular Physics
The spectroscopy and dynamics of the smallest Criegee intermediate CH2OO following UV excitation ... more The spectroscopy and dynamics of the smallest Criegee intermediate CH2OO following UV excitation to the B state is studied theoretically, based on multireference electronic wave functions and a qua...
The Fermi motion contriburion to quarkonia production at the Tevatron Run II
We show that the one order of magnitude discrepancy between the observed cross sections and calcu... more We show that the one order of magnitude discrepancy between the observed cross sections and calculational results for J/ψ may be improved partly by introducing the Fermi motion in its fragmentation production. We employ a light cone wave function and predict the J/ψ and the Υ singlet production at the Tevatron Run II and compare their differential as well as the total cross sections with and without the Fermi motion at this collider. Similar behavior is seen at the RHIC and at the LHC.
Advances in Chemical Research
In this work, we study the single and double ionization spectra of the M(CO)6,with M =( W and Cr ... more In this work, we study the single and double ionization spectra of the M(CO)6,with M =( W and Cr ) complexes by applying the four-component algebraic diagrammatic construction and Fock-space coupled cluster methods to extend earlier studies based on less demanding approaches. The computed single and double ionization potentials are in good agreement comparing with the available experimental results. The electronic structures of the cationic molecular systems are carefully investigated by computing accurately single and double ionization potentials. The final state characterization is relied on group theoretical considerations of the contributing orbitals and allowed for a clear assignment. Energy level diagrams show the effect of spin-orbit (SO) coupling starting from scalar relativistic results and for the heavy representative M(CO)6 with M =( W and Cr ) nonadditivity effects of the SO and electron correlation can be observed requiring a consistent treatment of both contributions.
The valence ionization spectrum of molybdenum hexacarbonyl: An ab initio quantum dynamical investigation
Chemical Physics Letters
Abstract The ionisation process corresponding to the 2 T 2 g ← 1 A 1 g transition in Mo ( CO ) 6 ... more Abstract The ionisation process corresponding to the 2 T 2 g ← 1 A 1 g transition in Mo ( CO ) 6 is studied by employing the nuclear quantum dynamical approach and electronic structure methods. The spectrum is simulated by construction of a Hamiltonian model. The computed spectrum is found to be in a good agreement with the corresponding experimental spectrum indicating the potential energy surfaces and time-dependent population analysis are correctly computed. We investigate the contribution of the spin-orbit coupling in separation of bands of the valence ionisation spectrum corresponding to the electronic states 1 E 5 / 2 + and 1 G 3 / 2 + by carrying out a trend analysis in series transition metal complexes M ( CO ) 6 , M = Cr , Mo , W .
Investigation of the valence ionization spectrum of chromium carbonyl using an ab initio quantum dynamical approach
The Journal of Chemical Physics
The nuclear dynamics of the chromium carbonyl cation following an ionization process correspondin... more The nuclear dynamics of the chromium carbonyl cation following an ionization process corresponding to the 2T2g ← 1A1g transition is studied theoretically, for the first time, using a fully quantal approach as well as high levels of the ab initio electronic structure and semiempirical density functional theory (DFT) methods. The photoelectron spectrum is calculated by the construction of a Hamiltonian model, in which the two totally symmetric modes ν19 (the Cr-C stretching mode) and ν39 (the C-O stretching mode) together with the spin-orbit (SO) coupling up to the zeroth-order SO splitting are treated. The potential energy curves along these two vibrational modes are computed by using the DFT. The simulated photoelectron spectrum is found to be in good agreement with the corresponding experimental one, leading to the conclusion that the potential energy surfaces and the diabatic population analysis are accurately determined. Our calculation confirms that the vibrational modes ν19 and ν39 are the vibrational progression of the valence ionization spectrum of the chromium carbonyl cation.
The ground state dynamics of s-trans-1,3-butadiene cation: An ab initio quantum dynamical study
Journal of Electron Spectroscopy and Related Phenomena
Abstract We used high levels of ab initio electronic structure methods and nuclear quantum dynami... more Abstract We used high levels of ab initio electronic structure methods and nuclear quantum dynamical approach for studying the ground state dynamics of the s-trans-1,3-butadiene cation. To benchmark the quality of the electronic structure calculations, potential energy surfaces for the energetically lowest lying doublet X2Bg, A2Au and B2Ag states along seven vibrational modes with considering mode S8 (asymmetric stretching C C bond) were calculated at the MRCI/CAS(10,7) and CAS(8,8) levels of theories. Considering contribution of S8 mode in the nuclear quantum dynamical investigation leads to a more realistic description of the ground state dynamics of s-trans-1,3-butadiene, which was not addressed before. Our calculation shows that the population transfer takes place in a time scale 50-60 fs. We also calculated the photoelectron spectrum of the molecular system under study. The excellent agreement of spectrum with the experimental one leads us the conclusion that the potential energy surfaces of three lowest cationic X2Bg, A2Au and B2Ag states and time-dependent population analysis of X2Bg were accurately determined.
The Journal of Chemical Physics
The excited state dynamics of the s-trans-1,3-butadiene cation, focusing on the second and third ... more The excited state dynamics of the s-trans-1,3-butadiene cation, focusing on the second and third bands of the photoelectron spectrum, have been investigated using a fully quantal approach, for the first time. The five lowest electronic states X 2 Bg, A 2 Au, B 2 Ag, C 2 Bu, and D 2 Ag considering the six vibrational modes S 3 , S5, S 13 , S 17 , S 19 , and S 20 were taken into account in the nuclear quantum dynamical investigation. The potential energy curves have been calculated along these coordinates for the five lowest electronic states using the RS2C method. Our simulations indicate a moderately fast population transfer from the A 2 Au and B 2 Ag to the X 2 Bg state, taking place on a time scale of 70-80 fs. Furthermore, the computed second and third bands of the photoelectron spectrum are in good agreement with the corresponding experimental ones. Our calculation identifies the symmetric stretching of the central C− −C bond S5 and the symmetric C− −C− −C bending S 13 to be the main progression modes of the second and third bands of the photoelectron spectrum of (cationic) s-trans-1,3-butadiene.
An ab initio quantum dynamical analysis of the vibronic structure of the X2Bg photoelectron spectral band of s-trans-1,3-butadiene
Chemical Physics
Abstract The nuclear dynamics of the s-trans- 1 , 3 -butadiene cation following a fast ionization... more Abstract The nuclear dynamics of the s-trans- 1 , 3 -butadiene cation following a fast ionization process is studied theoretically for the first time, using a fully quantal approach. The three lowest coupled X 2 B g , A 2 A u and B 2 A g states are taken into account and up to six nuclear degrees of freedom, including out-of-plane dihedral angles, are included. The underlying potential energy surfaces have been computed at RS2C level, a CASPT2 variant, and widely different CAS spaces have been evaluated beforehand. In the dynamics simulation, a small population transfer from the X 2 B g to the A 2 A u and B 2 A g states has been observed on a time scale 30–40 fs. The vibronic structure of the first band of the simulated photoelectron spectrum is in very good agreement with the experimental one. Our calculation reveals the vibrational mode corresponding to the symmetric stretching of the terminal C C bond to dominate the vibrational progression of the first band of the photoelectron spectrum of s-trans- 1 , 3 -butadiene.
A four-component Fock-space coupled cluster investigation of the XMn(CO)5, (X=Cl, Br and I) photoelectron spectra
Chemical Physics, 2016
Abstract In this work we readdress the theoretical interpretation of the XMn(CO) 5 , X = Cl, Br, ... more Abstract In this work we readdress the theoretical interpretation of the XMn(CO) 5 , X = Cl, Br, I photoelectron spectra by applying four-component Fock-space coupled cluster methods for their calculation. The final state characterization was based on group theoretical considerations of the contributing metal and ligand orbitals and the applied electronic structure methods extend earlier studies based on less demanding approaches. Energy level diagrams show the effect of spin–orbit (SO) coupling starting from scalar-relativistic results and especially for the heavy representative IMn(CO) 5 a sizeable influence of the iodine p spinors on the spectral features and nonadditivity effects of SO and electron correlation contributions could be observed.
2T2g ← 1A1g photo-electron spectrum of octahedral tungsten hexacarbonyl
Phys. Chem. Chem. Phys., 2016
The 2T2g ← 1A1g photo-electron spectrum of octahedral tungsten hexacarbonyl W(CO)6 is investigate... more The 2T2g ← 1A1g photo-electron spectrum of octahedral tungsten hexacarbonyl W(CO)6 is investigated quantum dynamically.
Quantum dynamics study of singlet–triplet transitions in s-trans-1,3-butadiene
Chemical Physics Letters, 2016
Abstract The intersystem crossing dynamics of s-trans -1,3-butadiene in its lowest singlet and tr... more Abstract The intersystem crossing dynamics of s-trans -1,3-butadiene in its lowest singlet and triplet states is studied theoretically, employing a fully quantal approach for the first time. The electronic states 2 1 A g , 1 1 B u , 1 3 B u and 1 3 A g , which interact vibronically and via the spin-orbit coupling are treated in the calculation, thus covering the lowest spin-forbidden electronic transitions. Up to five nuclear degrees of freedom, including out-of-plane dihedral angles are included in our investigation. The calculation of potential energy surfaces relies on the CASPT2 method, and the evaluation of spin-orbit coupling matrix elements using the full two-electron Breit–Pauli Hamiltonian is performed by utilizing the MRCI wavefunction. The latter dependence on the nuclear coordinates is included for the first time. An electronic population transfer on the sub-picosecond time scale due to intersystem crossing is obtained, a mechanism that can contribute to the singlet–triplet transitions in the electron energy loss spectrum of s-trans -1, 3-butadiene. It is found that the dependence of the spin-orbit coupling on the out-of-plane coordinates plays a dominant role in these singlet–triplet transitions. The amount of population transfer to the 1 3 A g and 1 3 B u states is roughly of the same order of magnitude.
A four-component Fock-space coupled cluster investigation of the HM(CO)5, (M = Mn, Re) photoelectron spectra
Molecular Physics, 2015
In this work, we readdress the photoelectron spectra of the HM(CO)5, (M=Mn, Re) carbonyl complexe... more In this work, we readdress the photoelectron spectra of the HM(CO)5, (M=Mn, Re) carbonyl complexes by applying four-component Fock-space coupled cluster (FSCC) methods for their calculation in order to extend earlier studies based on less demanding approaches. The final-state characterisation was based on group theoretical considerations of the contributing orbitals and allowed for an unambiguous assignment. Energy level diagrams show the effect of spin–orbit (SO) coupling starting from scalar relativistic results and for the heavy representative HRe(CO)5 nonadditivity effects of SO and electron correlation can be observed requiring a consistent treatment of both contributions.
Efficient computation of adiabatic electronic populations in multi-mode vibronic systems: Theory, implementation, and application
The Journal of Chemical Physics, 2012
The effective-mode formalism developed earlier is applied to efficiently compute adiabatic electr... more The effective-mode formalism developed earlier is applied to efficiently compute adiabatic electronic populations of multi-mode vibronic systems. Two different versions of the formalism are utilized. In the first one, the effective modes are used as new vibrational basis, and the time-dependent wave function as well as populations are calculated numerically exactly. In the second variant, the hierarchy-of-modes formalism is applied as an approximation scheme which leads to accurate results when including typically 7-10 members of the hierarchy. While in the first version, the propagation of the wave packet becomes numerically tedious, the computation of the adiabatic populations is rendered possible at all. Through the hierarchy-of-modes formalism, both the propagation as well as computation of adiabatic populations are speeded up by 1-3 orders of magnitude in typical cases. The formalism is applied to study the dynamics of pyrazine-type models for varying vibronic coupling strength and a (very) large number of bath modes.
Efficient computation of adiabatic populations in multi-mode Jahn-Teller systems through the use of effective vibrational modes
The Journal of Chemical Physics, 2011
A highly efficient scheme for computing adiabatic electronic populations in multi-mode Jahn-Telle... more A highly efficient scheme for computing adiabatic electronic populations in multi-mode Jahn-Teller systems is presented. It relies on the transformation to an effective-mode vibrational basis in which the relevant quantities depend on the coordinates of a single mode only. In this way, the generally tedious numerical evaluation of high-dimensional integrals is avoided and replaced by one-dimensional integrations. The effective-mode scheme is applied to a variety of two-mode and three-mode Jahn-Teller systems and gives a typical speedup of about two to three orders of magnitude as compared to the direct evaluation of the adiabatic populations. The gain grows rapidly with the numbers of modes.
JETP Letters, 2008
With the advent of high-precision frequency combs that can bridge large frequency intervals, new ... more With the advent of high-precision frequency combs that can bridge large frequency intervals, new possibilities have opened up for the laser spectroscopy of atomic transitions. Here, we show that laser spectroscopic techniques can also be used to determine the ground-state g factor of a bound electron: Our proposal is based on a double-resonance experiment, where the spin state of a ground-state electron is constantly being read out by laser excitation to the atomic L shell, while the spin flip transitions are being induced simultaneously by a resonant microwave field, leading to a detection of the quantum jumps between the ground-state Zeeman sublevels. The magnetic moments of electrons in light hydrogen-like ions could thus be measured with advanced laser technology. Corresponding theoretical predictions are also presented.
In dieser Arbeit werden mogliche Hochprazisionsmessungen des g-Faktors von gebundenen 1S Elektron... more In dieser Arbeit werden mogliche Hochprazisionsmessungen des g-Faktors von gebundenen 1S Elektronen untersucht und die bedeutendsten systematischen Effekte, die die Hochpr azisionsspektroskopie im ultravioletten und sichtbaren Spektralband beeinflussen, analysiert. Um den g-Faktor des gebundenen 1S-Elektrons eines in einer Penning-Falle gefangenen 4He Ions zu messen, werden zwei Anregungsschemata, die auf einer doppelresonanten elektronischen Anregung aufbauen, vorgeschlagen. Das erste Anregungsschema beruht auf der Anregung des 1S1/2(mj = +1/2) 2P3/2(mj = +3/2)-Ubergangs in einem 4HeIon durch zirkular polarisierte Ultraviolettstrahlung. Der angeregte Zustand 2P3/2(mj = +3/2) geht wegen seiner kurzen Lebenszeit in den Grundzustand uber und strahlt dabei ein Fluoreszenzphoton ab. Das Heliumion durchlauft diesen Kreislauf in der Falle und kann dabei jedesmal aufgrund des abgestrahlten Photons nachgewiesen werden. Gleichzeitig lost ein resonantes Mikrowellenfeld eine Umdrehung des Spin...
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Papers by behnam nikoobakht