Coherent Two-Dimensional Fourier Transform Infrared

Coherent Two-Dimensional Fourier Transform InfraredSpectroscopyM.Khalil,N.Demirdöven and A.TokmakoffDepartment of Chemistry,Massachusetts Institute of Technology,Cambridge,MA02139Telephone:(617)-253-4503,Fax:(617)-253-7030,E-mail:tokmakof@Abstract:Purely absorptive two-dimensional(2D)infrared vibrational spectra are obtained byadding together rephasing and non-rephasing2D spectra.The modulation of cross-peaks andemergence of new peaks are demonstrated as a function of the mixing period.2000Optical Society of AmericaOCIS codes:(300.6340)Spectroscopy,infrared;(300.6530)Spectroscopy,ultrafastTwo-dimensional(2D)infrared(IR)vibrational spectroscopy has recently been employed to study the structure and correlated dynamics of strongly coupled C≡O vibrations in metal dicarbonyl compounds and to determine the structure of small peptides in solution[1-5].Femtosecond heterodyne-detected third-order mid-infrared responses obtained as a function of an initial preparation period and a final detection period are Fourier transformed to give a 2D IR rmation on transient and evolving molecular structures is encoded in the position,amplitude, and shape of cross peaks in the2D IR spectrum.From a practical perspective,the use of this method requires the ability to obtain intuitive and physically relevant spectra with purely absorptive lineshapes.This study demonstrates the technique of2D Fourier transform(FT)IR spectroscopy using two different third-order non-linear signals to obtain purely absorptive features in the2D vibrational spectrum of the carbonyl stretches of dicarbonylacetylacetonato rhodium(I)(RDC)dissolved in hexane.The method is then used to explore the effects of relaxation on the various features in the2D vibrational spectra by introducing a variable mixing period.2D FT IR spectroscopy uses three field-matter interactions with the three input fields E a(k a),E b(k b)and E c(k c) arranged in a box-car geometry to generate a third-order non-linear polarization in the phase-matched directionk s=−k a+k b+k c.Two different signals are obtained at k s which differ only in the time ordering of the k a and k b pulses:S I=−k1+k2+k3(rephasing)and S II=+k1−k2+k3(non-rephasing).This technique is experimentally realized using a five-beam IR interferometer with90fs pulses centered at4.9µm.The incoming IR beam is split into4equal parts,three of which are used as input beams while the fourth beam is further split into a tracer and a local oscillator(LO).The tracer beam is sent along the path of the signal and is blocked during data collection. After the sample,the signal beam is spatially and temporally overlapped with the LO and dispersed in a monochrometer onto a64channel MCT detector.Rephasing and non-rephasing heterodyned signals are collected as a function ofτ1(delay between the first two pulses)for a particular mixing period,τ2(delay between the second and third pulses)by moving retroreflectors mounted on stepper-controlled translation stages.To determine the timing between pulses a and b to withinλ/10,the pulses are recombined after the sample and dispersed in a second monochrometer.Interference fringes collected as the timing between the two pulses is varied are used to calibrate theτ1axis.Fourier transforming along the correctedτ1axis leads to symmetric features in the resulting2D spectra.It is known from NMR,that collecting only the rephasing or the non-rephasing signals yields lineshapes that have mixed-phase character resulting in both absorptive and dispersive features in the real part of the2D spectrum. Pure absorptive lineshapes can be obtained by adding together the rephasing and non-rephasing2D spectra.Figure 1shows the rephasing and non-rephasing spectra with the phase-twisted lineshapes and their addition yielding pure absorptive features.The sum of the two responses shows eight features arising from the various ladder diagrams contributing to the third-order signals for this six-level system.We see that the diagonal and the cross-peaks consist of two features with opposite signs,separating the dynamics of the ground and first excited states.To correct for the phase mismatch between the signal and the LO fields arising due to pathlength differences,we phase the spectra by fitting the projection of the2D absorption spectrum alongω3to a dispersed pump-probe signal taken with the tracer as the probe under the same conditions[6].Figure2demonstrates the effect of a finiteτ2mixing time on the2D rephasing spectra.The first two panels show the modulation of the cross-peak intensity as a function ofτ2.We see that the intensity of the cross-peak reaches the maximum in Fig.2(a)whenτ2=nπ/2ωas where n is an even integer andωas is the frequency splitting between the symmetric and asymmetric carbonyl stretches.The cross-peaks disappear when n is an odd integer as shown in Fig.2(b).The third panel obtained whenτ2=7050fs indicates the emergence of four new features as a200020502100-ω1/2πc (cm -1)200020502100200020502100(c)(b)(a)-ω1/2πc (cm -1)ωsωaωa ωs ωsωa ωa ωsω3/2πc (c m -1)200020502100ω1/2πc (cm -1)Fig.1.Real part of 2D IR vibrational spectra at τ2=0(a)S I (b)S II and (c)S I +S II .result of various coherent and incoherent population relaxation processes occurring during the mixing time.This results in the diagonal and cross-peaks splitting into three features instead of the usual two features obtained at smaller values of τ2.A systematic study of the 2D rephasing and non-rephasing spectra as a function of τ2allows us to map out the complete dynamics of this multi-level system including the effects of solvent-induced relaxation and populationrelaxation.ωs ωaω3/2πc (c m -1)-ω1/2πc (cm -1)Fig.2.Absolute value 2D IR rephasing spectra as a function of a variable mixing time.(a)τ2=470fs (b)τ2=705fs(c)τ2=7050fs.1.O.Golonzka,M.Khalil,N.Demirdöven,and A.Tokmakoff,“Coupling and orientation between anharmonic vibrations characterized by two-dimensional infrared vibrational spectroscopy,”J.Chem.Phys.,115,10814-10828(2001).2.N.Demirdöven,M.Khalil,O.Golonzka,and A.Tokmakoff,“Correlation effects in two-dimensional vibrational spectroscopy of coupled vibrations,”J.Phys.Chem.A,105,8025-8030(2001).3.D.E.Thompson,K.A.Merchant and M.D.Fayer,“Two-dimensional ultrafast infrared vibrational echo studies of solute-solvent interactionsand dynamics,”J.Chem.Phys.,115,317-330(2001).4.M.T.Zanni,S.Gnanakaran,J.Stenger,and R.M.Hochstrasser,“Heterodyned two-dimensional infrared spectroscopy of solvent-dependent conformations of acetylproline-NH2,”J.Phys.Chem.B,105,6520-6535(2001).5.S.Woutersen,and P.Hamm,“Structure determination of trialanine in water using polarization sensitive two-dimensional vibrational spectroscopy,”J.Phys.Chem.B,104,11316-11320(2000).6.J.D.Hybl,A.Albrecht Ferro and D.M.Jonas,“Two-dimensional Fourier transform electronic spectroscopy,”J.Chem.Phys.,115,6606-6622(2001).。