Physical Chemistry, Molecular, and Materials Spectroscopy

spanglermontana.edu

There are two main areas of research being conducted in the Spangler group. The first involves investigation of optical materials and the mechanisms by which they function. In materials such as laser materials, optical power limiters (materials with non-linear absorption which can protect against lasers) and photorefractives, complicated energy transfer and charge transfer processes can occur after the initial photoexcitation. These processes can occur on timescales ranging from 10-13 sec to minutes and can cause spectral changes anywhere in the optical range (UV to IR). For this reason, we are developing new, multi-dimensional spectroscopic techniques for the investigation of optical materials that yield high information content in a relatively short experiment time.

For example, a commercially viable laser material must absorb at a wavelength where convenient light sources are available, undergo relaxation to a lower excited energy level with an acceptable rate, and have a slower subsequent emission from that level so that a population inversion can be established. To investigate potential laser materials, it is desirable to acquire simultaneously the emission intensity as a function of time and emission frequency. An example of this sort of 3-D spectrum from a crystalline laser material is shown in figure 1.

HO3+ YAG emission after pumping into the 5F5 band


In the study of optical power limiters, the desired information is the change in absorption caused by the photo excitation. We have developed fourier transform techniques to acquire time-resolved, photo-induced absorption spectra on time scales from picoseconds (10-12s) to minutes. Figure 2 shows the time-resolved photoinduced absorption spectrum of C60 in solution after pumping with a 6 ns 532 nm laser pulse. The initial feature is due to absorption by the triplet state of C60. The features that grow in at later times are due to charge transfer species.


Time-resolved photo-induced absorption spectrun of C60


Our second research area involves large amplitude, low frequency motions in non-rigid molecules and how they are affected by electronic changes. These vibrations typically have multiple minima potentials and can involve tunneling through the potential barrier. For these reasons, this type of motion does not conform to the standard quantum mechanical approximations used to describe vibrations. They cannot be described by the harmonic oscillator approximation and they cannot be classified using point group symmetry.

In addition to non-standard theoretical treatments, there are special experimental concerns as well. At room temperature, considerable spectral congestion results from thermal population of the low lying energy levels of these motions. Simple cryogenic cooling would reduce the congestion but would also condense the sample and perturb the large amplitude motion. This problem is solved by cooling the sample in a supersonic expansion which results in isolated, gas phase molecules exhibiting rotational and vibrational population distributions equivalent to molecules at 2 - 50K.


Remote substituent effects on methyl internal rotation barriers.
Studies of non-rigid molecules provide unique information as well as the special challenges mentioned above. For example, in methyl containing molecules, the barrier to internal rotation of the methyl group is very sensitive to changes in the π electron system. This is because hyperconjugation is a dominant influence on the methyl behavior. Work in our group dramatically demonstrates this point. The transitions due to methyl torsion in p-methyl-trans-stilbene shift to significantly lower frequencies when an electron donating group is substituted on the remote ring (see figure 3). The methyl barrier is 150 cm-1 in the excited state of p-methyl-trans-stilbene but lowers to 55 cm-1 in p\'-amino-p-methyl-trans-stilbene. This is nearly a factor of three change caused by substitution 10 carbons away. These studies may ultimately improve our understanding of π electron distribution and chemical reactivity in electronically excited states.

Selected Publications

Han,Y., Spangler,L. H., Hutcheson,R., and Equall, R. W.:
Photo-Induced Effects In Mn4+:Yag. Observation Of Unusually Efficient Excited State Absorption and a Long - Lived Metastable State
Materials Research Society Fall 1999 Conference Proceedings, In Press

Han, Y., Sonnenberg, W., Short, K., and Spangler, L.H.:
Fourier Transform Techniques for Measuring Absorption of Transient Species in Optical Limiting Materials
Proceedings of SPIE,3798, In Press

Farris, B., Smith, A., Martoglio, R., and Spangler, L.H.:
High Resolution Time and Frequency Resolved Spectroscopy for the Study of Photophysical Processes in Luminescent Materials
Materials Research Society Spring 1999 Conference Proceedings, In Press

Metzger, B. S. and Spangler, L.H.,:
Structural Information From Methyl Rotors : Methyl Torsional Barriers in p-Hydroxy-p\'-Methyl-t-Stilbene and Its Water Complexes
J. Phys.Chem. A.,101, 5431 (1997)

Spangler, L.H.:
Structural Information From Methyl Rotor Spectroscopy
Annu. Rev. Phys. Chem.,48, 475 (1997)

with Farris, B., Filer, E.D., and Barnes, N.P.:
A Computational Study of Host Effects on Er3+ Upconversion and Self-Quenching Efficiency in Ten Garnets
J. Appl. Phys.,79, 573 (1996)

Spangler, L.H. and Hutcheson, R.L.:
A Multi-dimensional Spectroscopic Facility for the Characterization of Laser Materials
Advanced Solid-State Lasers, San Francisco, CA January 31 - February 2, 1996

with D.W. Pratt in Jet Spectroscopy and Molecular Dynamics, J. M. Hollas and D. Phillips, eds.:
Internal Rotation Dynamics From Electronic Spectroscopy in Supersonic Jets and Beams
Blackie Academic and Professional, Glasgow 1995

Keywords:
Physical

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