Highly Excited Molecules - American Chemical Society

Chapter 13

Downloaded by NORTH CAROLINA STATE UNIV on January 10, 2013 | http://pubs.acs.org Publication Date: June 10, 1997 | doi: 10.1021/bk-1997-0678.ch013

A New Method for Measuring Vibrational Energy Distributions of Polyatomics 1

2

Derek R. McDowell , Fei Wu, and R. Bruce Weisman

Department of Chemistry and Rice Quantum Institute, Rice University, Houston, TX 77005

A new kinetics-based method is described for determining the distribution of vibrational energy contents in a highly excited, precollisional polyatomic sample. The method is suitable for excited species whose population decay constant depends on vibrational energy. In such cases a sample's decay kinetics will show a distribution of rate constants that can reveal the underlying initial distribution of vibrational energies. This method is applied to low pressure pyrazine samples whose total T population is measured by triplet-triplet transient absorption. The resulting highly defined kinetic data are analyzed to obtain a decay constant distribution, using a sum of Gaussian peaks model. The result is then transformed into a vibrational energy distribution using an independently determined calibration curve. The sample's narrow nascent energy distribution appears to evolve into a bimodal form through collisions with helium atoms. It is estimated that approximately 0.7% (on average) of the gas kinetic encounters between an excited triplet pyrazine molecule and a helium atom lead to vibrational deactivation of ca. 2000 cm . 1

-1

For many decades, physical chemists have recognized that vibrational excitation can be a crucial factor in controlling unimolecular and bimolecular processes of polyatomic molecules. An issue of central concern in systems activated either by thermal or optical excitation is the collisional flow of vibrational energy to and fromthe reactants. In recent years, "direct" methods have been developed that allow the average vibrational energy of relaxing samples to be monitored through molecular Current address: Department of Chemistry, Wittenberg University, P.O. Box 720, Springfield, OH 45501 Corresponding author © 1997 American Chemical Society In Highly Excited Molecules; Mullin, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1997.

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infrared emission, ultraviolet-visible absorption, or kinetic characteristics. Although these methods have provided unprecedented insight into vibrational energy transfer from highly excited polyatomics, they measure only the first moment of the sample's full vibrational energy distribution. This distribution, which describes the inhomogeneous variation in energy contents among the sample molecules, is of deeper interest than its first moment. New experimental information about such distributions has recently become available from extensions of the infrared emission method to deduce the second, as well as first, moment (7,2) and from the KCSI energy-selected photoionization technique of Luther and co-workers (3), which can reveal full energy distributions during the approach to thermalization. We describe here a new approach for deducing the distribution of vibrational energy among molecules in an excited, pre-collisional sample. This method, which we term "vibrational energy distributions from kinetic analysis," or " V E D K A , " is an outgrowth of our previous studies of average collisional energy loss from triplet state pyrazine. As illustrated schematically in Figure 1, optical excitation of selected vibrational levels of pyrazine's Sj state is followed by efficient, subnanosecond intersystem crossing that produces the Tj state with a well-defined amount of vibrational excitation equal to the difference between the photon energy and the Ί origin. These activated triplet state molecules subsequently undergo a second, much slower intersystem crossing process that returns them to the ground electronic state. Key to our methods is the strong dependence of the rate constant for this second process on the vibrational energy content of the triplet pyrazine molecule (4,5). Having constructed a "calibration curve" representing this dependence, we are able to assess average energy loss by analyzing the kinetics of total triplet population (measured through triplet-triplet transient absorption). The research reported here focuses on the population decay kinetics of low-pressure triplet pyrazine samples, with the goal of deducing not the average energy content, but instead the full vibrational energy distribution in the sample molecules. Our V E D K A method attempts to analyze kinetic data as superpositions of first-order rate constants that reflect the inhomogeneous variation of energy contents in the sample. Once the decay constant distribution has been experimentally deduced, it is mapped into the corresponding distribution of molecular vibrational energies. Our data confirm that the nascent energy distribution in the triplet pyrazine sample is much narrower than a thermal distribution of equal average energy. Although the V E D K A approach is rigorously valid only in the pre-collisional limit, we have found intriguing changes in the decay constant distribution caused by early collisions with helium atoms. These results suggest that the pyrazine energy distribution is quickly converted into an unexpected bimodal form through a surprisingly efficient collisional relaxation channel. ι

Experimental Method Our measurements were made on low pressure static samples of pyrazine vapor contained in a 68 cm optical cell. Sample molecules were excited to specific vibrational levels of the S state using tunable ultraviolet pulses generated by frequency-doubling the output of a Lumonics Q-switched NdrYAG / dye laser system. Following excitation, sub-nanosecond intersystem crossing leaves the excited pyrazine molecules in the T electronic state with a vibrational energy content of 4056 or {

{

In Highly Excited Molecules; Mullin, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1997.


13. MCDOWELL ETAL.

Vibrational EnergyDistributions ofPolyatomics 193

Pyrazine Triplet Relaxation

Figure 1. Schematic energy level diagram for pyrazine showing excitation and probe transitions, rapid intersystem crossing from Sj to T slower, energydependent intersystem crossing from Ί to S , and collisional relaxation within Tj. b

χ

0

In Highly Excited Molecules; Mullin, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1997.

194

HIGHLY EXCITED MOLECULES

5433 cm- (for excitation of the O Q or 8ag transition). The total triplet pyrazine


1

population is measured as a function of delay after excitation through its weak absorption of a continuous 676 nm diode laser probing beam, which co-propagates at a slight angle to the excitation beam in order to avoid transient absorption signals from the suprasil cell windows. A baffled and spectrally filtered silicon photodiode (EG&G model FND-100Q) detects the probe beam after it emerges from the sample cell. This photodiode's output waveform is recorded at 1 ns intervals and averaged over several thousand excitation pulses with a Tektronix TDS-744A digitizing oscilloscope. The resulting averaged traces are transmitted to a laboratory computer for conversion into induced absorbance and subsequent kinetic analysis. The instrumental response function is accurately determined by measuring the excitation pulse's shape with the detection system normally used for the probe beam. Random noise in the induced absorbance traces is reduced by the use of an amplitude-stable probe laser, a detection bandwidth limit of 20 MHz, and repetitive signal averaging. Avoidance of systematic errors is more difficult, requiring rigorous suppression of electronic and optical cross-talk between the excitation and probe systems. Our optical setup is also designed to reduce artifacts from thermal lensing in the sample as well as from cell window transients. The instrument's performance is checked with empty-cell scans, which typically show systematic and random induced absorbance errors below ca. 2 χ 10 . For most sample conditions, peak signal to r.m.s.-noise ratios reach several hundred. -6

Data Analysis The central target of the data analysis is /Ε{Ε, ή, a function whose energy-dependence at a given time describes the inhomogeneous distribution of vibrational energies in the triplet population and whose integral over energy gives the total Tj population at time t. We note that the T

Highly Excited Molecules - American Chemical Society

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