Thomas J. Preston, Maitreya Dutta, Brian J. Esselman, Michael A. Shaloski, Robert J. MCMahon, and F. Fleming Crim

UW-Madison

Aimable Kalume, Lisa George, and Scott A. Reid

University of Marquette

Isomerization Between CH2ClI and CH2Cl-I in Cryogenic Matrices Studied on Ultrafast

Timescales

Isomerization of HalomethanesFirst identified in

solid matrices with frequency-domain spectroscopy*

Return to parent following electronic excitation

Stable in cryogenic solids

Transient in liquids

*Maier…Hess, JACS, 112, 5117 (1990).

Experiment

Frequency Domain Time Domain

500:1 Pulsed deposition

Tunable, Nd:YAG nanosecond dye laser

Static, difference spectroscopy

220:1 Continuous deposition

Ultrafast, Ti:Sapphire laser

Transient absorption spectroscopy

Potential Energy Sketch

Transient Absorption - Photolysis

Transient Absorption - Probe

Transient Absorption - Recovery

Spectroscopy of Parent and Isomer

Spectroscopy of Parent and Isomer

Pump and Probe Wavelengths

Probe Vibrational Energy with Electronic Transition

Probe Vibrational Energy with Electronic Transition

Probe Vibrational Energy with Electronic Transition

Isomer Absorption In Ar, N2, CH4

Transient Absorption: CH4

Transient Absorption: CH4

Transient Absorption: CH4

Transient Absorption: CH4

Rise time, t1 / ps

1.1(2)

1.4(1)

2.3(3)

3.1(3)

Transient Absorption: CH4

Rise or fall time, t2 / ps

42(7)

276(122)

49(13)

37(5)

Potential – 1D

Isomer Formation and Relaxation

t1: 1 ps

t2: 40 ps

MP2/Sadlej-pVTZ

Trends Same in CH4, N2, and Ar

Complete PictureVibrationally excited isomer forms in about

1 psVibrational relaxation occurs in about 40 psBoth of these timescales are similar in

liquid and super-critical fluids

Unanswered QuestionsWhat is the detailed process of formation

(t1)?Formation time is 2x faster in N2 and CH4

than in ArWhat is the detailed process of cooling (t2)?

Rates are indiscriminate of matrix complexityRates similar among solids, liquids, super-

critical fluidsOpportunity for MD simulations

AcknowledgementsScott Reid and group at Marquette

Static spectroscopyCalculations

Bob McMahon and group at UW-MadisonMatrix isolation apparatusCalculations

Fleming Crim and group at UW-Madison

End of slide show, click to exit.

Thomas J. Preston, Maitreya Dutta, Brian J. Esselman, Michael A. Shaloski, Robert J. McMahon, and F. Fleming Crim

UW-Madison

Aimable Kalume, Lisa George, and Scott A. ReidUniversity of Marquette

Isomerization of CH2Cl-I to CH2ClI in Cryogenic Matrices:

A Study on Ultrafast Timescale

Recap: Formation of the Isomer

Explore Potential Energy Surface

a

Setup

Probe

Preparation

Pump

Destruction

Wall-Clock Time

Isom

er

Popu

lati

on

Probe Isomer or Parent?

Probe Isomer or Parent?

Prep.

Pump Probe

Destruction of Isomer

lpump = 400 nm

lprobe = 400 nm

lpump = 800 nm

lprobe = 400 nm

IR Pump – C-H Stretch Overtone

Loss to Parent

VibrationalRelaxation

6000 cm-1

IR Pump – C-H Stretch Fundamental

3000 cm-1

VibrationalRelaxation

Vibrational Relaxation

Vibrational Relaxation

Vibrational Relaxation

Vibrational Relaxation

Excite C-H stretchEnergy flows into

Franck-Condon mode (t1)

Energy flows into surroundings (t2)

Infrared Excitation100 mm-thick samples10-20% conversion to isomerSmall absorption at C-H overtoneExceptionally large cross section at C-H

fundamental

AcknowledgementsScott Reid and group at Marquette

Static spectroscopyCalculations

Bob McMahon and group at UW-MadisonMatrix isolation apparatusCalculations

Fleming Crim and group at UW-Madison