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Molecular electronic,  vibrational and rotational motion in optical and x-ray fields
KTH, School of Biotechnology (BIO), Theoretical Chemistry (closed 20110512). (Theoretical Chemistry)
2009 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The subject of this theoretical  study is the role ofelectronic structure as well as of rotational and vibrational motionson interactions between molecules and electromagnetic radiation,ranging from optical to x-ray. The thesis concerns both linear and nonlinear regimes of the light-matter interaction. The first part of the thesis is devoted to propagation of opticalpulses with different time-structure through various nonlinear absorbers.First we explain the double-exponential decay of fluorescence caused by photobleaching of pyrylium  salt irradiated by a train of short (100 fs) optical pulses. The main reason for this effect is the transversal inhomogeneity of the light beam which makes the dynamics of the photobleaching differ in the core of the pulse and on its periphery. We also explore the optical power limitingof C60 fullerene irradiated by either microsecond optical pulses or a picosecond pulse trains. Enhancement of nonlinear absorption is caused by strong triplet-triplet absorption that becomes important due toelongation of the interaction time.Here we show the importance of the repetitionrate for the optical power limiting performance.The second part of the thesis addresses the interaction of optical and x-rayfields with rotational degrees of freedom of molecules. In this part the main attention is paid to the rotational heating caused by the recoil, experienced by molecules due to the ejection of photoelectrons. We have quantitatively explained two qualitatively different experiments with the N2 molecule.We predict the interference modulation of the recoil-induced shift,which is a shift of the photoelectron line caused by the rotational recoil effect, as a function of the photon energy.The developed theory also explains the rotational heating ofmolecules observed in the optical fluorescence induced by x-ray radiation.Based on this explanation, we suggest a new scheme of the optical fluorescence induced by x-rays that allows to detect the recoil effect via the recoil-inducedsplitting of the optical resonance.The last part of the thesis focuses on multi-mode nuclear dynamics of the resonant Auger scattering from the C2H2 molecule, that was the subject of a recent experimental study.Here we develop a theory that explains the observed vibrationalscattering anisotropy. We have found that three qualitatively different mechanisms are responsible for this phenomenon. The first mechanism is the interference of the direct and resonance scattering channels. The second mechanismis the interference of the resonant scattering channels through core excitedstate with the orthogonal orientation of the vibrational modes of core excitedstate. The Young's double slit like interference of the quantum pathways through the double-well potential of the bending motion of core excited state is the third mechanism of the vibrational scattering anisotropy.

Place, publisher, year, edition, pages
Stockholm: KTH , 2009. , viii, 67 p.
Series
Trita-BIO-Report, ISSN 1654-2312 ; 2009:24
Keyword [en]
x-ray, nonlinear optics, photobleaching, recoil, anisotropy, Auger
National Category
Atom and Molecular Physics and Optics
Identifiers
URN: urn:nbn:se:kth:diva-11386ISBN: 978-91-7415-483-2 (print)OAI: oai:DiVA.org:kth-11386DiVA: diva2:275164
Public defence
2009-12-01, FB 53, AlbaNova, Roslagstullsbacken 21, Stockholm, 10:00 (English)
Opponent
Supervisors
Note
QC 20100713Available from: 2009-11-10 Created: 2009-11-03 Last updated: 2011-11-23Bibliographically approved
List of papers
1. Many-photon dynamics of photobleaching
Open this publication in new window or tab >>Many-photon dynamics of photobleaching
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2007 (English)In: Journal of Physical Chemistry A, ISSN 1089-5639, E-ISSN 1520-5215, Vol. 111, 11961-11975 p.Article in journal (Refereed) Published
Abstract [en]

A detailed dynamical theory of photobleaching by periodical sequences of laser pulses is presented. The theory is used for interpretation of recent experiments with pyrylium salts. Our simulations are based on first-principles simulations of photoabsorption cross-sections and on empirical rate constants. Two competitive channels of photobleaching, namely, photobleaching from the lowest excited singlet and triplet states and from higher excited states, are found to explain different intensity dependences of the photobleaching rates in different samples. The process includes two-photon excitation from the ground state to the first or second excited singlet states and one-photon excitation from the first singlet or triplet states to higher excited states. The fluorescence follows double-exponential dynamics with two characteristic times. The first and the shorter one is the equilibrium settling time between the ground and the lowest triplet states. The second characteristic time, the time of photobleaching, is responsible for the long-term dynamics. The effective rate of photobleaching from the first excited singlet and lowest triplet states depends differently on the irradiance in comparison with the photobleaching in higher states. The first channel is characterized by a quadratic intensity dependence in contrast to the second channel that shows a cubic dependence. The competition between these photobleaching channels is very sensitive to the rate constants as well as to the repetition rate, the pulse duration, and the peak intensity. The double-exponential decay of the fluorescence is explained by the spatial inhomogeneity C of the light beam. The findings in this work are discussed in terms of the possibility of using many-photon-induced photobleaching for new three-dimensional read-write devices.

Keyword
DENSITY-FUNCTIONAL THEORY; MOLECULAR-ORBITAL METHODS; GAUSSIAN-TYPE BASIS; 2-PHOTON ABSORPTION; PHOTOPHYSICAL PROPERTIES; FLUORESCENCE MICROSCOPY; 2-STEP PHOTOLYSIS; ORGANIC-MOLECULES; TIME; EXCITATION
National Category
Industrial Biotechnology
Identifiers
urn:nbn:se:kth:diva-6997 (URN)10.1021/jp074756x (DOI)000251140700003 ()2-s2.0-36949023529 (Scopus ID)
Note
QC 20100713Available from: 2007-04-20 Created: 2007-04-20 Last updated: 2017-12-14Bibliographically approved
2. Optical limiting and pulse reshaping of picosecond pulse trains by fullerene C60
Open this publication in new window or tab >>Optical limiting and pulse reshaping of picosecond pulse trains by fullerene C60
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2009 (English)In: Journal of Electron Spectroscopy and Related Phenomena, ISSN 0368-2048, E-ISSN 1873-2526, Vol. 174, 125-130 p.Article in journal (Refereed) Published
Abstract [en]

We present a dynamical theory of nonlinear absorption and propagation of a laser pulse train that contains 20 subpulses with an individual pulse width of loops. It is shown that the accumulative nonlinearity and the reverse saturation absorption play important roles in the optical limiting performance and pulse shaping. When the incident field is not too strong, the population transfer reveals a slow response process, and the periodic sequence of short light pulses can be regarded as a continuous long pulse. The general theory is applied to fullerence C-60, which is a popular reverse saturable absorption material and a good limiter because of its larger excited-state absorption cross-section compared with that of the ground state. The propagation of the front subpulses is mainly affected by the linear transition between the ground state and the first excited singlet state, while the latter subpulses are attenuated by the excited-state absorption. Moreover, these two different kinds of absorption mechanisms result in different radial distributions for different subpulses. The pulse propagation is studied by solving numerically the coupled rate equations and the propagation equation of the optical pulse intensity, using experimental parameters as input. We suggest a new method to measure the lifetime of the triplet state.

Keyword
Pulse trains; Optical limiting; Fullerence; Singlet-triplet
National Category
Industrial Biotechnology
Identifiers
urn:nbn:se:kth:diva-11435 (URN)10.1016/j.elspec.2009.03.010 (DOI)000271347700021 ()2-s2.0-70349209130 (Scopus ID)
Note
QC 20100713Available from: 2009-11-10 Created: 2009-11-10 Last updated: 2017-12-12Bibliographically approved
3. Optical limiting for microsecond pulses
Open this publication in new window or tab >>Optical limiting for microsecond pulses
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2009 (English)In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 130, no 5, 054114Article in journal (Refereed) Published
Abstract [en]

We present a dynamical theory of nonlinear absorption and propagation of laser pulses with duration in the microsecond time domain. The general theory is applied to fullerene C-60 because of its good optical limiting properties, namely, a rather low ground state absorption and a strong triplet-triplet absorption. It is shown that sequential absorption involving strong triplet-triplet transitions is the major mechanism of nonlinear absorption. The intrinsic hierarchy of time scales makes an adiabatic solution of the coupled rate equations valid, which therefore can be reduced to a single dynamical equation for the ground state population. The slow evolution of this population is defined by an effective rate of population transfer to the triplet state and by the pulse duration. The propagation effect plays an important role in the optical power limiting performance. The intensity of the field as well as the population of the triplet state decreases during the pulse propagation, and a weakened nonlinear sequential two-photon absorption is followed by a linear one-photon absorption which gradually becomes the dominating process. The competition between these qualitatively different processes depends on the field intensity, the length of the absorber, and the concentration. The pulse propagation is studied by solving numerically the two-dimensional paraxial field equation together with the effective rate equation for the ground state population.

Keyword
atomic clusters, fullerenes, ground states, laser beam effects, molecule-photon collisions, optical limiters, triplet state, two-photon processes
National Category
Industrial Biotechnology
Identifiers
urn:nbn:se:kth:diva-11434 (URN)10.1063/1.3072560 (DOI)000263167100017 ()2-s2.0-59949102448 (Scopus ID)
Note

QC 20100713

Available from: 2009-11-10 Created: 2009-11-10 Last updated: 2017-12-12Bibliographically approved
4. Photoelectron-recoil-induced rotational excitation of the B 2 state in N2+
Open this publication in new window or tab >>Photoelectron-recoil-induced rotational excitation of the B 2 state in N2+
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2009 (English)In: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 79, no 022506Article in journal (Refereed) Published
Abstract [en]

In the photoelectron spectrum of N-2 the apparent ionization energy to form the B (2)Sigma(+)(u) state increases linearly with the photon energy. Rotationally resolved measurements of the fluorescent decay of this state show a linear increase of rotational heating with increasing photon energy. These results are in quantitative agreement with the prediction of the theory of recoil-induced rotational excitation, indicating that the rotational heating that has been observed previously arises primarily from such recoil-induced excitation. Together with other results that have been reported they show that recoil-induced internal excitation is significant in many situations, including near threshold.

National Category
Industrial Biotechnology
Identifiers
urn:nbn:se:kth:diva-11436 (URN)10.1103/PhysRevA.79.022506 (DOI)000263815000071 ()2-s2.0-61649092877 (Scopus ID)
Note

QC 20100713

Available from: 2009-11-10 Created: 2009-11-10 Last updated: 2017-12-12Bibliographically approved
5. X-ray induced rotational heating of N2
Open this publication in new window or tab >>X-ray induced rotational heating of N2
(English)Manuscript (preprint) (Other academic)
National Category
Industrial Biotechnology
Identifiers
urn:nbn:se:kth:diva-14108 (URN)
Note
QC 20100713Available from: 2010-07-13 Created: 2010-07-13 Last updated: 2010-07-15Bibliographically approved
6. Recoil splitting of x-ray-induced optical fluorescence
Open this publication in new window or tab >>Recoil splitting of x-ray-induced optical fluorescence
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2010 (English)In: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 81, no 3, 035401Article in journal (Refereed) Published
Abstract [en]

We show that the anisotropy of the recoil velocity distribution of x-ray-ionized atoms or molecules leads to observable splittings in subsequent optical fluorescence or absorption when the polarization vector of the x rays is parallel to the momentum of the fluorescent photons. The order of the magnitude of the recoil-induced splitting is about 10 mu eV, which can be observed using Fourier or laser-absorption spectroscopic techniques.

Keyword
Photoion Rotational Distributions
National Category
Industrial Biotechnology
Identifiers
urn:nbn:se:kth:diva-14110 (URN)10.1103/PhysRevA.81.035401 (DOI)000276262500238 ()2-s2.0-77949744532 (Scopus ID)
Funder
Swedish Research CouncilCarl Tryggers foundation
Note

QC 20100713

Available from: 2010-07-13 Created: 2010-07-13 Last updated: 2017-12-12Bibliographically approved
7. May which-path detection trigger vibrational anisotropy of resonant Auger scattering?
Open this publication in new window or tab >>May which-path detection trigger vibrational anisotropy of resonant Auger scattering?
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(English)Manuscript (preprint) (Other academic)
National Category
Industrial Biotechnology
Identifiers
urn:nbn:se:kth:diva-14111 (URN)
Note
QC 20100713Available from: 2010-07-13 Created: 2010-07-13 Last updated: 2010-07-15Bibliographically approved

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