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Raman Scattering at Resonant or Near-Resonant Conditions: A Generalized Short-Time Approximation
KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology.
KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology.
KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology.
KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology.ORCID iD: 0000-0002-1763-9383
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2012 (English)In: Chinese Journal of Chemical Physics, ISSN 1674-0068, Vol. 25, no 1, 31-47 p.Article in journal (Refereed) Published
Abstract [en]

We investigate the dynamics of resonant Raman scattering in the course of the frequency detuning. The dephasing in the time domain makes the scattering fast when the photon energy is tuned from the absorption resonance. This makes frequency detuning to act as a camera shutter with a regulated scattering duration and provides a practical tool of controlling the scattering time in ordinary stationary measurements. The theory is applied to resonant Raman spectra of a couple of few-mode model systems and to trams-1,3,5-hexatriene and guanine-cytosine (G-C) Watson-Crick base pairs (DNA) molecules. Besides some particular physical effects, the regime of fast scattering leads to a simplification of the spectrum as well as to the scattering theory itself. Strong overtones appear in the Raman spectra when the photon frequency is tuned in the resonant region, while in the mode of fast scattering, the overtones are gradually quenched when the photon frequency is tuned more than one vibrational quantum below the first a,absorption resonance. The detuning front the resonant region thus leads to a strong purification of the Raman spectrum from the contamination by higher overtones and soft modes and purifies the spectrum also in terms of avoidance of dissociation and interfering fluorescence decay of the resonant state. This makes frequency detuning a very useful practical tool in the analysis of the resonant Raman spectra of complex systems and considerably improves the prospects for using the Raman effect for detection of foreign substances at ultra-low concentrations.

Place, publisher, year, edition, pages
2012. Vol. 25, no 1, 31-47 p.
Keyword [en]
Resonant Raman, Scattering duration, Hexatriene, Short time approximation
National Category
Physical Sciences Chemical Sciences
Identifiers
URN: urn:nbn:se:kth:diva-28338DOI: 10.1088/1674-0068/25/01/31-47ISI: 000304380900005Scopus ID: 2-s2.0-84863386740OAI: oai:DiVA.org:kth-28338DiVA: diva2:386755
Funder
Swedish e‐Science Research Center
Note

QC 20120618. Updated from manuscript to article in journal.

Available from: 2011-01-13 Created: 2011-01-13 Last updated: 2013-04-08Bibliographically approved
In thesis
1. Theoretical Studies of Raman Scattering
Open this publication in new window or tab >>Theoretical Studies of Raman Scattering
2011 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Different theoretical approaches have been presented in this thesis to study the Raman scattering effect. The first one is response theory applied up to third order of polarization, where the determination of α, β and γ is used to calculate linear Raman scattering (resonance Raman scattering (RRS) and normal Raman scattering (NRS)), hyper Raman scattering (HRS) and coherent anti-Stokes Raman scattering (CARS), respectively. The response theory refers to adiabatic time-dependent density functional theory in the complex domain with applications on RRS and NRS, and to a recently developed methodology (Thorvaldsen et al. [105, 106]) for the analytic calculation of frequency-dependentpolarizability gradients of arbitrary order, here with applications on CARSand HRS. Various systems have been studied with the response theory, such as explosive substances (DNT, TNT, RDX and H2O2), optical power limiting materials (platinum(II) acetylide molecules), DNA bases (methylguanine-methylcytosine) and other systems (Trans-1,3,5-hexatriene and Pyridine). We have explored the dependency of the calculated spectra on parametrization in terms of exchange-correlation functionals and basis sets, and on geometrica loptimization.

The second approach refers to time-dependent wave packet methodology for RRS and its time-independent counterpart in the Kramers-Heisenberg equation for the scattering cross section, which reduces the calculation of the RRS amplitude to computation of matrix elements of transition dipole moments between vibrational wave functions. The time-dependent theory has been used to examine RRS as a dynamical process where particular attention is paid to the notion of fast scattering in which the choice of photon frequency controls the scattering time and the nuclear dynamics. It is shown that a detuning from resonance causes a depletion of the RRS spectrum from overtones and combination bands, a situation which is verified in experimental spectra.

The cross section of NRS has been predicted for the studied molecules to be in the order of 10−30 cm2/sr. A further increase in sensitivity with a signal enhancement up to 104 to 105 is predicted for the RRS technique, while CARS conditions imply an overall increase of the intensity by several orders of magnitude over NRS. In contrast to RRS and CARS, the HRS intensity is predicted to be considerably weaker than NRS, by about four orders of magnitude. However, silent modes in NRS can be detected by HRS which in turncan provide essential spectroscopic information and become complementary to NRS scattering.

With the above mention methodological development for NRS, RRS, CARS and HRS, we have at our disposal a powerful set of modelling tools for the four different Raman techniques. They have complementary merits and limitations which facilitate the use of these spectroscopes in applications of Raman scattering for practical applications, for instance stand-off detection of foreign substances.

Place, publisher, year, edition, pages
Stockholm: KTH, 2011. ii, 66 p.
Series
Trita-BIO-Report, ISSN 1654-2312 ; 2011:2
National Category
Theoretical Chemistry
Identifiers
urn:nbn:se:kth:diva-28332 (URN)978-91-7415-844-1 (ISBN)
Public defence
2011-01-28, FD51, AlbaNova, Universitetscentrum, Stockholm, 08:40 (English)
Opponent
Supervisors
Funder
Swedish e‐Science Research Center
Note
QC 20110112Available from: 2011-01-13 Created: 2011-01-13 Last updated: 2012-05-24Bibliographically approved

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