Multi-Dimensional IR Spectroscopy of Acetic Acid Dimers and Liquid Water

Transcription

1 Multi-Dimensional IR Spectroscopy of Acetic Acid Dimers and Liquid Water N. Huse 1, J. Dreyer 1, E.T.J.Nibbering 1, T. Elsaesser 1 B.D. Bruner 2, M.L. Cowan 2, J.R. Dwyer 2, B. Chugh 2, R.J.D. Miller 2 1 Max-Born-Institute for Nonlinear Optics und Short Pulse Spectroscopy Berlin, Germany 2 Department of Chemistry and Physics University of Toronto, Canada Partially funded by SFB 450

2 Outline Multi-dimensional spectroscopy Couplings in acetic acid dimers Structural dynamics in pure water

3 From Linear to Multi-Dimensional Spectroscopy Linear spectroscopy 2D spectroscopy T Which transitions couple to each other? What is the broadening mechanism? How fast are the dynamics? Excitation ν 1 Detection ν 3 Signals of anharmonic oscillators Fundamental transition Excited state absorption Cross peaks coupling Excitation ν T Detection ν 3

4 Heterodyne Detected Photon Echoes T τ 3 τ 1 klo, -k1 + k2 + k3 Infrared detector array ν = 8cm -1 k LO Monochromator Spectrograms for different delays of τ T 1 LO τ 3 τ τ = τ 3-1 ν 3 ν 3 ν3 Field extraction Fourier Pulse delay transform S(τ 1,ν 3 ) E(τ 1,ν 3 ) E(ν 1,ν 3 )

5 Acetic Acid Dimers Cyclic acetic acid dimer OH-stretching vibration well-defined geometry Absorbance (OD) Phenol in C 2 Cl 4 Acetic Acid in CCl 4 2 levels vs. multi-level structure Wavenumber (cm -1 v OH =1 v=1,i red-shift of ν(o-h) v OH =0 v=0,i O H O H O very strong broadening structured bandshape

6 Acetic Acid Dimers Cyclic acetic acid dimer OH-stretching vibration ip bending 148 cm -1 ip stretching -143 cm -1 Absorbance (OD) Phenol in C 2 Cl 4 Acetic Acid in CCl 4 2 levels vs. multi-level structure Wavenumber (cm -1 v OH =1 v OH =0 v=1,i v=0,i O H O H O Broadening mechanism: anharmonic coupling to low-frequency modes Fermi resonances? Heyne et al., J. Chem. Phys. 121 (2004), 902

7 Fermi Resonances Fermi resonances ν OH 1 δ OH 2 (ν OH /2δ OH ) g (ν OH /2δ OH ) u Excitation Frequency ν T=0fs T=400fs Detection Frequency ν 3-1 Signal Amplitude (norm.) 1 δ OH =1 0 0 ν OH,δ OH =0 Transitions: ν OH, δ OH, ν CO, ν C=O T=0fs T=400fs Profiles Detection Frequency ν Absorbance (OD)

8 Ab-initio Calculations Theory Experiment IR Intensity (arb.u.) Linear Nonlinear Linear Nonlinear 1 Excitation Frequency ν T=400 fs T=400 fs Detection Frequency ν 3 Detection Frequency ν 3 Ab-initio calculations: Fermi resonances dominate 2D spectra Cubic coupling constants with i = ν bu OH j (a g ) δoh νc O δoh k (b u ) νc O δoh δoh Dreyer, Int. J. Quantum Chem. (in press) νc=o νc O -85

9 Liquid Water Hydrogen bonds determine the structure and other properties of water MD simulation of water (from Parinello group) Inverse Absorption Length Linear absorption spectrum Wieliczka ' Frequency Wielczka et al., Appl. Opt. 28 Fluctuating geometry HOD in D 2 O: Stenger et al., Phys. Rev. Lett (2001) Stenger et al., J. Phys. Chem A 105 (2001), 2929 OH stretching OH bending Librations

10 Ultrafast Timescales in Liquid Water O-H stretching vibration Librations O-H stretching modes (3400 cm -1 ) O-H bending mode (1650 cm -1 ) Fast librations ( 1500 cm -1 ) Vibrational lifetimes ~ 0.01 ps ~ 0.02 ps ~ 0.03 to 0.1 ps ~ 0.2 ps Microscopic dynamics of hydrogen bonds is ultrafast.

11 Frequency-Resolved Transient Grating T 1 3 LO 2 τ 1 =0fs Spectral diffusion signal decay at the edges within 50 fs Energy transfer anisotropy decay in less than 100 fs Pulse delay 50fs Population relaxation OH stretch lifetime = 190 fs

12 Correlation Spectra of Pure Water 3500 T=0fs T=50fs Pulse sequence ν τ 1 T=0 fs T=50 fs ν LO ν Microscopic origin: librational modes with ω > ω(o...o) Inverse Absorption Length Wielczka et al., Appl. Opt. 28 OH bending Librations Frequency

13 Conclusions Heterodyne photon echo spectroscopy delivers full information on couplings and line broadening mechanism Fermi resonances dominate 2D spectra of acetic acid although cubic coupling constants are similar for coupling to low-frequency modes (Huse et al., submitted) Loss of inhomogeneity in liquid water within 50 fs due to librational modes of the hydrogen bond network, T fs (Cowan et al., Nature 434 (2005), 199)