Time-Resolved Frequency Comb Spectroscopy of Transient Free Radicals in the Mid-IR

Bryce J Bjork, Adam J. Fleisher, Bryan Changala, Thinh Quoc Bui, Kevin Cossel,Mitchio Okumura, Jun Ye

ISMS, June 16, 2014

JILA, NIST and University of Colorado at Boulder

Introduction

• Transient Radical Chemistry– The OH+CO Reaction and the HOCO Radical

• Our Mid-IR Frequency Comb Spectrometer– Time-Resolved Comb Spectroscopy

• DOCO Radical Chemistry– Experiment– Results

Transient Radical Chemistry

Smith, I. & Zellner, R. Rate Measurements of Reactions of OH by Resonance Absorption. (1973).

Atmospheric Chemistry: Recycles OH to HO2 inthe atmosphere.

Combustion Chemistry: Primary route forconversion of CO to CO2. Major fraction of heat released.

not linear…(non-Arrhenius)

trans- and cis-HOCO Intermediates

Francisco, J. S., Muckerman, J. T. & Yu, H.-G. HOCO radical chemistry. Acc. Chem. Res. 43, 1519–26 (2010).

Optical Frequency Combs can help!

Transient Radical Chemistry

Radicals are difficult to study!

Produced in small concentrations Requires high sensitivity

Short Lifetimes Requires high time resolution

Many chemicals participate in reaction Requires broad bandwidth

Time (log scale)Wavenumber

grating

Mid-IR Camera

Mid-IR Comb Spectroscopy

1. Mid-IR Frequency Comb Source2. Optical Enhancement Cavity3. Spatially Dispersive VIPA Spectrometer

(1)(2)(3)VIPA etalon

The Mid-IR Comb Source

1/136 MHz

fo +Df

fo

f

Fourier Transform

𝑓 =𝑛 𝑓 𝑟𝑒𝑝+ 𝑓 0

Centered at 3.7 μm

Bandwidth~ 300 nm

1. Mid-IR Frequency Comb Source

10 W Combat 1 μm

SynchronouslyPumped OPO

Mid-Infrared Radiationtunable between 3-5μm

(1)(2)(1)

(2)

DOCOBand

The Interaction Cell

M1M2

N2 purgeN2 purge

193 nm Photolysis Beam

Chemical Interaction Region

2. Optical Enhancement Cavity

The chemical reaction:

193 nm +Acrylic Acid-d1 DOCO Vinyl Radical

Deuterated to avoid water lines…

VIPA Detection

3. Spatially Dispersive VIPA Spectrometer

N2 reference 0.2% N2O in N2 at 40 torrCavity finesse ~1000

InSb Camera

Lots of Temporal Information

Ref

time

Sig 1

Sig2

Sig3

Excimer Pulse Camera IntegrationWindow4 ms

Looking for 100 us signal

DOCO Experiment Results

trans-DOCO Simulation

HDO Simulation

D2O Simulation

DOCO Experiment Results

D2O, HDO, C2HDLong Timescale Dynamics

trans-DOCO at Short Timescales

DOCO Experiment ResultsD

iffus

ion Acrylic Acid Depletion

Long Timescale Dynamics

193 nm

+Acrylic Acid-d1

propynal (?) HDO

193 nm+

Acrylic Acid-d1

formic acid (?) C2HD

Isot

ope

Exch

ange

D2O

HOD

Short Timescale DynamicsC2HD and HDO unexpected! They appear immediately following the excimer pulse.

Therefore, they are possibly formed from unimolecular dissociation of acrylic acid.

Fleisher, A. J. et al. Mid-Infrared Time-Resolved Frequency Comb Spectroscopy of Transient Free Radicals. J. Phys. Chem. (2014).

Aleksandra Foltynowicz, Piotr Maslowski, Florian Adler, Oliver HecklJILA|NIST and CU

Lora Nugent-Glandorf, Florian Adler, Tyler Neely, Scott A. DiddamsNational Institute of Standards and Technology

Acknowledgements