Tải bản đầy đủ - 0 (trang)
IV. SUMMARY AND FUTURE PROSPECTS

IV. SUMMARY AND FUTURE PROSPECTS

Tải bản đầy đủ - 0trang

534



Koehl et al.



a few optical elements, almost no beam alignment steps, and little or no

searching for signal location.

All of the advances discussed here have evolved quite recently,

and their exploitation is far from complete. For example, experiments on

phonon-polaritons in other classes of materials such as semiconductors, thin

films, and multilayer assemblies will be improved and facilitated by the use

of these methods. Coherent optical control and manipulation of phononpolaritons with spatially and temporally shaped excitation fields has only

just begun, with the use of spatiotemporal imaging to direct the excitation

field parameters. This may lead to the use of phonon-polaritons as coherent

signal carriers for terahertz bandwidth signal processing applications. In

addition, ISRS-excited phonon-polaritons provide a source of conveniently

tunable terahertz frequency radiation that emerges from the crystal in which

it was produced. Such radiation has been passed through a sample and

detected optically after reentry into the crystalline medium, permitting

frequency-tunable, narrowband terahertz spectroscopy through the use of

visible light, an extremely compact apparatus, and no far-IR optics (28).

Finally, the methods reviewed here have related applications

in acoustic wave excitation and monitoring and in transient grating

measurements in general. Extremely facile delivery of excitation and probe

beams to the sample, detection of signal, and switching among excitation

wavevectors have simplified transient grating experimentation to the level

of pump-probe transient absorption measurements. At the same time,

capabilities for high wavevector definition, linearization of the signal with

respect to the material response, assessment of grating spatial phase, and

separation of different signal components including those from phase and

amplitude grating contributions have improved and added to the information

content that can be extracted.

The recent advances are bringing about a broadening of the transient

grating experimental user community and the range of areas under study,

as well as a deepening of the knowledge that can be gained in each area.



APPENDIX: PHONON-POLARITON EXCITATION: EQUATIONS

OF MOTION



The equations of motion describing the transverse optic phonon and electromagnetic modes (modeled as oscillators with charged masses, whose

displacements lie along the x direction and whose wavevectors lie mainly

along the y direction in the yz plane), their coupling, and their responses



Copyright © 2001 by Taylor & Francis Group, LLC



Lattice Vibrations



535



to light are as follows (7,19,2931):

mi xă i C ki xi x D kie xi

me xă e C ke xe x D kie xi



xe 2 x C qi E

xe 2 x C qe E



ε q, ω ∂2 E C 4 P

c2

∂t2

Pe D e E D Nqe xe x



rðrðED



Pi D



iE



D Nqi xi x



6

7

8

9

10



In these equations, xi represents displacements of the polar optic phonon of

mass mi , charge qi , force constant ki , and number density N along the crystal

axis parallel to the unit vector x; xe represents electronic displacements with

the lowest electronic resonance characterized by mass me , charge qe , and

force constant ke near the Brillouin zone center; E is the electric field

of the terahertz radiation; and Pi and Pe are the polarizations induced in

the dielectric by the phonon and electronic responses, with P D Pi C Pe .

The corresponding linear contributions to the susceptibility tensor

are

i and

e . Without any nonlinear coupling terms, the solutions to the

phonon-polaritons are propagating electromagnetic modes with the dispersion shown, for example, in Fig. 1. These contributions to the susceptibility

may be combined with other contributions to yield a perturbative expansion

of the complex dielectric permittivity ε q, ω . In many cases, it is possible

to simplify the system into sets of optical and far-infrared modes of the

form of Eq. (8) with couplings between the modes based on the coupling

term in Eqs. (6) and (7).

This coupling term kie xi xe 2 is a phenomenological approximation of the lowest-order (cubic) potential energy terms that gives rise to

nonlinear effects. These effects can be described by perturbative expansions

of (7,30,32,33):

ω12 Pi ω1 C ωi2 Pi ω1 D ωi2 ε0 f i E ω1 C

iii ω1 , ω2 , ω3

iee ω1 , ω2 , ω3 C 2 iie ω1 , ω2 , ω3 E ω2 E ω3 g

ω12 Pe ω1 C ωe2 Pe ω1 D ωe2 ε0 f e E ω1 C eii ω1 , ω2 , ω3

C eee ω1 , ω2 , ω3

2 eie ω1 , ω2 , ω3 E ω2 E ω3 g



11

12



The effect of the kie x2e term on the motion of xe is written as eee , for

example, and physically represents optical second harmonic generation

(SHG), to which the polar phonons do not contribute because they are

not able to drive or follow high-frequency responses. This nonlinearity



Copyright © 2001 by Taylor & Francis Group, LLC



536



Koehl et al.



should have roughly the same magnitude at terahertz frequencies, as can

be seen through Miller’s rule (30,34), because the electrons can still follow

the terahertz fields. Two other perturbative terms out of the four possible

symmetry-reduced terms are interesting. (Unfortunately, one limitation of

this phenomenological analysis is that the fourth term eii is not particularly

intuitive in the Born-Oppenheimer approximation.) Garrett first noted (30)

that iee represents Raman scattering, in which two optical fields drive

a lower-frequency resonance by an electronic deformation of the ionic

potential; this term has also been described as an inverse electro-optic

effect (6,7). Finally, the intriguing term iii represents the all-ionic contribution to terahertz-frequency SHG, which occurs at frequencies low enough

for both phonons and electrons to contribute to and be affected by the

nonlinearity. It is also possible to estimate contributions to this process

through a quantum-mechanical perturbation theory and wavevector overtone spectroscopy (35,36). The clearest measurements of iii so far (with

some contamination by iie ) have been made at sub-terahertz frequencies on

a number of materials, including LiTaO3 , BaTiO3 , and GaAs (32,33). The

corresponding terahertz SHG coefficients are enormous compared to SHG

coefficients at optical frequencies due to the substantial ionic susceptibilities

in linear response that contribute to both “driving” and “following.”

The dielectric is often assumed to be isotropic in order to simplify

Eq. (8) by assuming transverse phonon-polaritons; the extension to

anisotropic media is straightforward (31). In the limit of very short pulse

duration compared to the phonon-polariton oscillation period, the timedependence of the excitation field can be treated as a delta function, and

the phonon-polariton response is given by the impulse response function

for the spatial excitation pattern used. If crossed excitation pulses are used,

then it is simplest to describe the excitation and response in terms of the

excitation wavevector or wavevector range.

It is straightforward to see that the phonon-polaritons generated by

crossed excitation pulses will have a wavevector component qz in the

forward (z) direction in addition to the expected wavevector component

in the optical grating (y) direction. Note that the excitation pulses arrive

first at the front of the sample, then the middle, then the back, so the vibrational phase varies linearly as a function of depth into the crystal (the z

direction). In the limit of a very small scattering angle, i.e., a single excitation beam, there is no wavevector component at all in the y direction, and

a traveling wave with wavevector entirely in the z direction is produced

through forward ISRS. In general, and especially for small scattering angles,

the phonon-polariton phase fronts may be tilted substantially into the yz



Copyright © 2001 by Taylor & Francis Group, LLC



Lattice Vibrations



plane. The full wave vector is given by (19,36).

p

p

ωopt εopt

ωph εopt

k1 k2 C

k1 C k2

kph D š

c

2c



537



13



where k1 C k2 points in the z direction and k1 k2 points in the y direction. ωopt is the central frequency of the excitation pulses, k1 and k2 are

the central wavevectors of the pulses, ωph is the frequency of the excited

phonon-polaritons, and εopt is the dielectric permittivity at optical frequencies. Equation (13) follows from the twin requirements of energy and

momentum conservation (19,36). Although ωopt × ωph , for small angles

the second term cannot be neglected. Figure 10 shows a simulation of the

phonon-polariton response to crossed excitation pulses, illustrating the y

and z components of the wave vector.



Figure 10 Gray-scale plot of coherent phonon displacements A y, z, t . The excitation pulses form an interference pattern, which is moving in the Cz direction

(vertical). The displacement direction is along the x-axis (out of the page). The

y component of the grating wavevector, kg D 1000 cm 1 , is given by the excitation interference pattern, fringe spacing of 63 µm. The z component arises due to

the finite propagation time of the excitation pulses through the sample. The total

polariton wavevector magnitude is kp D 1153 cm 1 , corresponding to a wavelength

of 54 àm. (Adapted from Ref. 19.)



Copyright â 2001 by Taylor & Francis Group, LLC



538



Koehl et al.



REFERENCES

1.

2.



3.

4.

5.

6.



7.

8.



9.



10.



11.

12.



13.



14.

15.

16.

17.



Dhar L, Rogers JA, Nelson KA. Time-resolved vibrational spectroscopy in

the impulsive limit. Chem Rev 1994; 94:157.

Yan Y-X, Cheng L-T, Nelson KA. Impulsive stimulated light scattering. In:

RJH Clark, RE Hester, eds. Advances in Nonlinear Spectroscopy, Vol. 16.

Richester: Wiley, 1988:299–355.

Nelson KA, Miller RJD, Lutz DR, Fayer MD. Optical generation of tunable

ultrasonic waves. J Appl Phys 1982; 52(3):1144–1149.

Fayer MD. Dynamics of molecules in condensed phases: picosecond holographic grating experiments. Ann Rev Phys Chem 1982; 33:63–87.

Thomsen C, Grahn HT, Maris HJ, Tauc J. Surface generation and detection

of phonons by picosecond light pulses. Phys Rev B 1986; 34(6):4129–4138.

Auston DH, Cheung KP, Valdmanis JA, Kleinman DA. Cherenkov radiation

from femtosecond optical pulses in electro-optic media. Phys Rev Lett 1984;

53(16):1555–1558.

Auston DH, Nuss MC. Electrooptical generation and detection of femtosecond

electrical transients. IEEE J Quantum Electron 1988; QE-24(2):184–197.

Etchepare J, Grillon G, Antonetti A, Loulergue JC, Fontana MD, Kuge GE.

Third-order nonlinear susceptibilities and polariton modes in PbTiO3 obtained

by temporal measurements. Phys Rev B 1990; 41(17):12362.

Dougherty TP, Wiederrecht GP, Nelson KA. Impulsive stimulated Raman

scattering experiments in the polariton regime. J Opti Soc Am B 1992; 9(12):

p. 2179.

Wiederrecht GP, Dougherty TP, Dhar L, Nelson KA, Leaird DE, Weiner AM.

Explanation of anomalous polariton dynamics in LiTaO3 . Phys Rev B 1995;

51(2):916.

Maznev AA, Nelson KA, Rogers JA. Optical heterodyne detection of laserinduced gratings. Opt Lett 1998; 23(16):1319.

Rogers JA, Fuchs M, Banet MJ, Hanselman JB, Logan R, Nelson KA.

Optical system for rapid materials characterization with the transient grating

technique: application to nondestructive evaluation of thin films used in

microelectronics. Appl Phys Lett 1997; 71(2):225–227.

Goodno GD, Dadusc G, Miller RJD. Ultrafast heterodyne-detected transientgrating spectroscopy using diffractive optics. J Opt Soc Am B 1998;

15(6):1791.

Maznev AA, Crimmins TF, Nelson KA. How to make femtosecond pulses

overlap. Opt Lett 1998; 23:1378.

Koehl RM, Adachi S, Nelson KA. Real-space polariton wave packet imaging.

Chem J Phys 1999; 110(3):1317.

Koehl RM, Adachi S, Nelson KA. Direct visualization of collective

wavepacket dynamics. J Phys Chem 1999; 111:3559–3571.

Yan Y-X, Nelson KA. Impulsive stimulated light scattering. I. General theory.

J Chem Phys 1987; 87(11):6240–6256.



Copyright © 2001 by Taylor & Francis Group, LLC



Lattice Vibrations



539



18. Yan Y-X, Nelson KA. Impulsive stimulated light scattering. II. Comparison

to frequency-domain light-scattering spectroscopy. J Chem Phys 1987;

87(11):6257–6265.

19. Brennan CJ. Femtosecond wavevector overtone spectroscopy of anharmonic

lattice dynamics in ferroelectric crystals. Cambridge, MA: Massachusetts Institute of Technology, 1997.

20. Weiner AM, Leaird DE, Wiederrecht GP, Nelson KA. Femtosecond pulse

sequences used for optical manipulation of molecular motion. Science 1990;

247(4948):1317.

21. Koehl RM, Adachi S, Nelson KA. Multiple-pulse control and bispectral 2D

Raman analysis of nonlinear lattice dynamics. In: Ultrafast Phenomena XI. T

Elsaesser et al., eds. Berlin: Springer-Verlag, 1998:136–137.

22. Rogers JA, Maznev AA, Banet MJ, Nelson KA. Optical generation and characterization of acoustic waves in thin films: fundamentals and applications.

Ann Rev Mater Sci 2000; 30:115–157.

23. Gunter P, Huignard J-P. Photorefractive Materials and Their Applications I.

Berlin: Springer-Verlag, 1988.

24. Johnson J. Structural and dynamic origins of intensity in holographic relaxation spectroscopy. J Opt Soc Am B 1985; 2:317–321.

25. Adachi S, Koehl RM, Nelson KA. Real-space and real-time imaging of propagating polariton-wavepackets. Butsuri 1999; 54(5):357.

26. Born M, Wolf E. Principles of Optics. Oxford: Pergamon Press, 1980.

27. Hecht E. Optics. Reading, MA: Addison-Wesley, 1987.

28. Crimmins TF, Nelson KA. Phys Rev B, submitted.

29. Born M, Huang K. Dynamical Theory of Crystal Lattices. Oxford: Clarendon

Press, 1954.

30. Garrett CGB. Nonlinear optics, anharmonic oscillators, and pyroelectricity.

IEEE J Quantum Electron 1968; QE-4(3):70.

31. Barker AS, Loudon R. Response functions in the theory of Raman scattering

by vibrational and polariton modes in dielectric crystals. Rev Modern Phys

1972; 44:18.

32. Boyd GD, Bridges TJ, Pollack MA, Turner EH. Microwave nonlinear susceptibilities due to electronic and ionic anharmonicities in acentric crystals. Phys

Rev Lett 1971; 26(7):387.

33. Pollack MA, Turner EH. Determination of absolute signs of microwave nonlinear susceptibilities. Phys Rev B 1971; 4(12):4578.

34. Boyd RW. Nonlinear Optics. San Diego, CA: Academic Press, Inc., 1992.

35. Brennan C, Nelson KA. Direct time-resolved measurement of anharmonic

lattice vibrations in ferroelectric crystals. J Chem Phys 1997; 107:9691–9694.

36. Romero-Rochin V, Koehl RM, Brennani CJ, Nelson KA. Theory of

anharmonic phonon-polariton excitation in LiTaO3 by ISRS and detection by

wavevector overtone spectroscopy. J Chem Phys 1999; 111(8):3559–3571.



Copyright © 2001 by Taylor & Francis Group, LLC



13

Vibrational Energy Redistribution in

Polyatomic Liquids: Ultrafast IR-Raman

Spectroscopy

Lawrence K. Iwaki∗ , John C. De`ak† , Stuart T. Rhea‡ , and Dana D. Dlott

University of Illinois at Urbana–Champaign, Urbana, Illinois



I. INTRODUCTION



In this chapter we discuss recent measurements of vibrational energy redistribution in polyatomic liquids using the ultrafast IR-Raman technique (1,2).

In the IR-Raman technique, a tunable midinfrared (mid-IR) pulse, is used

to pump a selected vibrational transition of a polyatomic liquid, and a

time-delayed visible pulse is used to monitor the instantaneous vibrational

population via incoherent anti-Stokes Raman scattering. The energy redistribution processes that will be considered are vibrational energy relaxation

(VER) and vibrational cooling (VC) (3). VER refers to the elementary

process of energy loss from a specific vibrational mode (the “system”) to

some or all of the other mechanical degrees of freedom (the “bath”). VC

refers to the process where a vibrationally hot molecule loses its excess

vibrational energy to the surroundings. VC is not an elementary process like

VER. It is a complex process that generally involves many VER steps (4,5).

The VER and VC processes considered in this chapter will all involve

polyatomic liquids at ambient temperature. It is useful to contrast this problem

Ł







Current affiliation: National Institute of Standards and Technology, Gaithersburg, Maryland

Current affiliation: Procter & Gamble Company, Ross, Ohio

Current affiliation: CMI, Inc., Owensboro, Kentucky



Copyright © 2001 by Taylor & Francis Group, LLC



542



Iwaki et al.



with two fundamentally different problems that have been studied to a greater

extent: VER of isolated molecules or high-pressure gases, for which a vast

literature exists, and VER of diatomic molecules in condensed phases.

Isolated polyatomic molecules can undergo only intramolecular

vibrational energy redistribution (IVR). Isolated molecules cannot lose their

vibrational energy except by (slow) radiative processes. IVR is a type

of vibrational dephasing process, where the initially prepared vibrational

excitation time-evolves into other isoenergetic states. Neglecting radiative

processes, gas-phase molecules can lose vibrational energy only through

collision, so VER in gases is usually associated with high-pressure gases

(6). In high-pressure gases, a target molecule heated with a laser is observed

to lose its energy by many binary collisions with a buffer gas. Many

authors have tried to draw a close analogy between VER in high-pressure

gases and VER in condensed phases (7). For example, at STP a hot gasphase molecule will experience collisions with a buffer gas at an average

frequency of ¾1010 s 1 , whereas at liquid densities a hot molecule will

experience collisions at an average frequency of ¾1013 s 1 . Theories that

try to model condensed phase VER as simply faster gas-phase VER are

termed “binary collision theories” (7). VER measurements have been made

that bridge the gap between gases and fluids, including the remarkable

works of Troe (8–10), Fayer (11–13), and Harris (14–17). After many

years of studying binary collision models, a consensus opinion is emerging

that binary collision models fail to capture the complexity of condensed

phase VER, because many-body interactions are intrinsically significant

and cannot be neglected (18,19).

Diatomic molecules are the simplest condensed phase VER systems,

for example, a dilute solution of a diatomic such as I2 or XeF in an atomic

(e.g., Ar or Xe) liquid or crystal. Other simple systems include neat diatomic

liquids or crystals, or a diatomic molecule bound to a surface. VER of

a diatomic molecule can occur only by energy transfer to the collective

vibrations of the bath, i.e., the phonons. Ordinarily VER is a high-order

multiphonon process. Consequently there is an enormous variability in VER

lifetimes, which may range from 56 s [liquid N2 (20)] to 1 ps [e.g., XeF in

Ar (21)], and a high level of sensitivity to environment. Diatomic molecules

have simple structures but complex VER mechanisms.

Polyatomic molecules represent a major step up in complexity because

every molecule has several vibrational modes. This feature guarantees enormous qualitative differences between diatomic and polyatomic VER and

casts doubt on the possibility of extrapolating insights gained from studying

diatomics to polyatomic systems. However, in what is at first glance a

paradox, polyatomic molecules have a complex structure but ordinarily

much simpler VER mechanisms than diatomic molecules. That is because

Copyright © 2001 by Taylor & Francis Group, LLC



Ultrafast IR-Raman Spectroscopy



543



the “ladder” VER process discussed in Section II, which often dominates in

polyatomics, is lower-order and thus much simpler than higher-order multiphonon processes involved in diatomic VER. A ladder process is one where

an excited vibration loses its energy to a lower energy vibration plus a small

number of phonons, via lower-order anharmonic couplings. The “rungs” of

the vibrational ladder are the vibrations of the polyatomic molecule.

VER occurs as a result of fluctuating forces exerted by the bath on the

system at the system’s oscillation frequency (22). We will use the upper-case

to denote the system’s vibrational frequency and lower-case ω to denote

other vibrations. It may also be useful to look at fluctuating forces exerted

on a particular chemical bond (23). Fluctuating forces are characterized by

a force-force correlation function. The Fourier transform of this force correlation function at , denoted Á

, characterizes the quantum mechanical

frequency-dependent friction exerted on the system by the bath (19,22).

This friction, especially at higher (i.e., vibrational) frequencies, plays an

essential role in condensed phase chemical reaction dynamics (24,25).

The multiple roles of VER (friction) and VC (dissipation) in

essentially all condensed-phase chemical processes have been extensively

discussed (18,19). In chemical reactions (Fig. 1), the “system” is the

specific mode of the reactant [or a coupled set of reactant and solvent

modes (19)] associated with the reaction coordinate. Chemical reactions

are “catalyzed” by vibrational energy. The system becomes activated by

vibrational energy from the bath. Then the barrier is crossed. Then that

vibrational energy plus the enthalpy of reaction is returned to the bath

(Fig. 1). Much has been written about the dynamical effects of VER on

barrier crossings (19,25,26). If VER is too slow, there is too little friction

and chemical reactions are slower due to multiple barrier recrossings. If

VER is too fast, there is too much friction and chemical reactions are

again slower because barrier crossing becomes slow. If the VER rate is

just right, the rate is the maximum possible. The maximum attainable rate,

which is exactly that given by transition-state theory (19,26), occurs at

the Kramers turnover (Fig. 1). The VER rate is systematically varied in

practice by pressure-tuning the solvent density, as in classic studies of

photoisomerization of stilbene (8,27–29) and boat-chair isomerization of

cyclohexane (30,31).

A major breakthrough in the measurement of VER occurred in 1972.

Laubereau et al. (32) used picosecond laser pulses to pump molecular

vibrations via stimulated Raman scattering (SRS) and time-delayed incoherent anti-Stokes probing to study VER of C–H groups in ethanol and

methanol ¾3000 cm 1 . Alfano and Shapiro (33) used the same technique to monitor both the decay of the initially excited (parent) C–H stretch

excitation and the appearance and subsequent decay of a daughter vibration,

Copyright © 2001 by Taylor & Francis Group, LLC



544



Iwaki et al.



Figure 1 (a) Vibrational energy catalyzes chemical reactions. The reactant R is

activated by multiphonon up-pumping, when R takes up the enthalpy of activation

H† from the bath. That energy plus the heat of reaction is returned to the bath

after barrier crossing. (b) VER influences chemical reaction rates. When VER is

just the right rate, the reaction rate is a maximum at the Kramers turnover. When

VER is too slow or too fast, the reaction rate decreases. (From Ref. 50.)



a C–H bending vibration ¾1460 cm 1 . Several reviews have described

these early studies of liquids (2,7,22,34,35).

Another important breakthrough occurred with the 1974 development

by Laubereau et al. (36) of intense tunable ultrashort mid-IR pulses. IR excitation is more selective and reliable than SRS, so SRS pumping is hardly

ever used any more. At present the most powerful methods for studying

VER in condensed phases use IR pump pulses. The most common (and

complementary) techniques to probe nonequilibrium vibrational dynamics

induced with mid-IR pump pulses are anti-Stokes Raman probing (the IRRaman method) or IR probing (IR pump-probe experiments).

In the early days of IR-Raman measurements (2,34) with older laser

technology [e.g., Nd:glass (2)], the technique was mainly limited to studying

energy leaving higher frequency parent vibrations pumped by mid-IR laser

pulses, such as C–H stretching transitions ¾3000 cm 1 , OH stretching

transitions ¾3600 cm 1 , or metal carbonyl C O stretching transitions

¾2000 cm 1 (34). In a few cases it was possible to observe the first

generation of high-frequency hv × kT daughter vibrations, e.g., energy



Copyright © 2001 by Taylor & Francis Group, LLC



Ultrafast IR-Raman Spectroscopy



545



transfer from parent C–H stretch ¾3000 cm 1 to daughter C C stretch

1968 cm 1 of acetylene (34,37), energy transfer from N–H stretch of

pyrrole (38) ¾3400 cm 1 in benzene solution to ring stretching modes

of pyrrole ¾1400 cm 1 or to the benzene solvent ¾1000 cm 1 , energy

transfer from C–H stretch to C–C stretching ¾1400 cm 1 of naphthalene

(39), and energy redistribution among nearly degenerate C O stretching

vibrations ¾2000 cm 1 of W(CO)6 (40).

Recent advances in laser instrumentation have, for the first

time, permitted researchers to monitor the redistribution of vibrational

energy throughout all Raman-active vibrations of a polyatomic molecule.

In noncentrosymmetrical molecules, that would be essentially all the

molecule’s vibrations. Dlott and coworkers (41), using a 300 Hz Nd:YLF

laser system, were the first to observe vibrational energy flow in a

polyatomic liquid from parent C–H stretching vibrations all the way

down to the lowest energy doorway vibrations in their studies of VER

in nitromethane (NM). However the time resolution of ¾30 ps was not

good enough to resolve all relevant VER processes. The same authors also

studied intermolecular vibrational energy transfer between alcohols and

nitromethane (42). Graener et al. used a 50 Hz Nd:YLF system with 1.5 ps

resolution to study VER in dichloromethane (43) CH2 Cl2 and chloroform

(44) CHCl3 after C–H stretch excitation. The latter work is especially

notable for being the first where every one of the vibrations of a particular

polyatomic molecule was monitored during a VER process. Recently our

group at the University of Illinois has developed an IR-Raman instrument

based on a Ti:sapphire laser, which provides 1 ps time resolution and

unprecedented sensitivity (45). This system has been used to investigate

VER in a variety of polar, nonpolar, and associated liquids and mixtures

(46–50). In particular, we have extensively investigated acetonitrile (ACN)

(46,47), which is widely viewed as a model for polyatomic liquids (18,51)

and throughout this chapter we will illustrate general concepts with specific

examples from our ACN studies.

In this chapter, we first discuss the theoretical framework needed to

understand VER measurements, including force-force correlation function

methods and perturbative techniques. We discuss experimental aspects of

the IR-Raman technique, paying attention to the laser instrumentation, the

experimental setup, the nature of the pumping and probing processes, detection sensitivity and optical background, and the interpretation of results

including spectroscopic artifacts. Then we provide examples from recent

research by our group, focusing on timely problems such as the vibrational cascade, the dynamics of doorway vibrations, methods for probing the



Copyright © 2001 by Taylor & Francis Group, LLC



Tài liệu bạn tìm kiếm đã sẵn sàng tải về

IV. SUMMARY AND FUTURE PROSPECTS

Tải bản đầy đủ ngay(0 tr)

×