Femtosecond Coherent Anti-Stokes Raman Spectroscopy (fs-cars) with Nitrobenzene and Nitomethane

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1 Proc Indan Natn Sc Acad 81 No. 2 March 2015 pp Prnted n Inda. DOI: /ptnsa/2015/v812/48105 Research Paper Femtosecond Coherent Ant-Stokes Raman Spectroscopy (fs-cars) wth Ntrobenzene and Ntomethane VINU V NAMBOODIRI 1, MOHAMMED AHMED 1, G KRISHNA PODAGATLAPALLI 2 and AJAY K SINGH 1* 1 Radaton and Photochemstry Dvson, Bhabha Atomc Research Centre, Mumba , Inda 2 Advanced Centre of Research n Hgh Energy Materals, Unversty of Hyderabad, Hyderabad , Inda (Receved on 30 November 2014; Revsed on 19 December 2014; Accepted on 5 January 2015) Tme-resolved coherent ant-stokes Raman Scatterng (CARS) s a nonlnear spectroscopc tool, whch has ganed mportance wth the development of ultrafast lasers due to ts ablty to probe ultrafast dynamcal processes n molecules. Apart from the tme resoluton, the superorty of CARS over spontaneous Raman scatterng derves from ts hgh sgnal generaton ablty and coherent, laser lke, sgnal emsson. In ths contrbuton, we descrbe the development of an expermental faclty for the tme-resolved coherent ant-stokes Raman scatterng spectroscopy and ts applcaton to study the vbratonal dynamcs of molecules contanng ntro (NO 2 ) groups. fs-cars experments were carred out n neat lquds of ntrobenzene, ntromethane and ther mxture. A detaled mappng of the dynamcs ntated by the stmulated Raman process s acheved from the spectrum of the transent CARS sgnal. The CARS transent shows rch beatng structure correspondng to frequences as hgh as 10 THz. The Fourer transform of the transent allows accurate determnaton of the vbratonal energy dfferences.ths methodology s capable of provdng the dephasng behavour and spectral nformaton of the molecular system at the same tme.the results demonstrate that CARS spectroscopy wth femtosecond tme resoluton and wavenumber-resolved detecton s a powerful tool for the characterzaton of dynamcs n the electronc ground state of molecules. Key Words: Femtosecond; CARS; Vbratonal Dynamcs; Ntromethane; Ntrobenzene Introducton Wth the applcaton of lasers, spontaneous Raman spectroscopy developed as a powerful analytcal tool and s appled n a wde varety of areas. However, due to nherently weak sgnal levels and fluorescence background, ts applcablty s restrcted when the sample concentraton s too low or the sample s fluorescent. These dffcultes assocated wth spontaneous Raman spectroscopy can be overcome by coherent Raman spectroscopy (Potma and Mukamel, 2013). Coherent Raman technques are non-lnear spectroscopc technques and provde much hgher sgnal levels compared to spontaneous Raman sgnals. Whle spontaneous Raman sgnal s emtted n all drectons, the coherent Raman sgnal s a laser lke emsson n a well-defned drecton, whch makes the detecton easer and background-free. Coherent Raman spectroscopc technques are now wdely beng used as an alternatve to conventonal Raman spectroscopy due to ts hgh sgnal levels and coherent nature of the sgnal. Coherent ant-stokes Raman scatterng (CARS) (Potma and Mukamel, 2013; El-Dasty, 2011) s one of the coherent Raman technques that has evolved as a versatle technque for materal charactersaton and magng n varous areas of scence. Though *Author for Correspondence: E-mal: aksngh@barc.gov.n; Tel:

2 526 Vnu V Namboodr et al. expermentally complex compared to conventonal Raman, the hgh sgnal levels and natural rejecton of fluorescence elmnates the most common factors plagung the applcablty of conventonal Raman spectroscopy. Frequency doman CARS, usng narrow band lasers, has proved to be an effcent vbratonal spectroscopc tool n low-pressure gases, combuston processes, plasma and also n molecular spectroscopy and mcroscopy (Djaker et al., 2007; Roy et al., 2010). The development of ultrafast lasers added another dmensonalty to laser spectroscopy. Tmeresolved spectroscopy usng ultrashort laser pulses of pco/femtosecond duraton made t possble to probe fast molecular dynamcal processes n real tme. The tme scales provded by ultrafast lasers correspond to the vbratonal perod of molecular vbratons (100 fs ~ 150 cm 1 ) and are deal for tme-resolved probng of vbratonal dynamcs n molecules. Most of the femtosecond tme-resolved studes have been performed on nvestgaton of dynamcs of molecular systems n electroncally excted states. Femtosecond tme-resolved CARS (fs-cars) s an expermental technque, whch s capable of studyng dynamcs n the electronc ground state of molecular systems. It was frst reported by probng ultrafast vbratonal dynamcs n several lquds (Leonhardt et al., 1987). Usng the broad spectral wdth of femtosecond pulses, several vbratonal modes can be excted coherently (wave packets) and can montor ther relaxaton processes n real tme. As a result of the ntense sgnal, senstvty and nherent florescence dscrmnaton, CARS s of consderable nterest n tme-resolved spectroscopy. Some of the applcatons of tmeresolved CARS nclude studes of vbratonal dephasng tmes n soluton, collsonal relaxaton tmes, and study of the excted state propertes (Schmtt et al., 1998; Verhelg et al., 1999).The large spectral wdth of fs pulses lmts the spectral resoluton acheved by fs-cars. Developments n pulse shapng technology (Wener, 2011; Monmayrant et al., 2010) provde a way to mprove the spectral resoluton n fs-cars. In ths contrbuton, we descrbe the development of an expermental faclty for tme-resolved CARS usng femtosecond laser pulses and ts applcaton n probng tme-resolved vbratonal dynamcs n ntrobenzene, ntromethane and ther mxture. Many of the hgh-energy materals contan ntro groups and the am of the CARS experment was to explore the applcablty of CARS n decpherng the electronc ground state vbratonal dynamcs of hgh energy materals. The paper s organsed as follows: next secton provdes a bref account of the theory of CARS followed by the expermental descrpton. The fnal secton dscusses the expermental results on ntromethane, ntrobenzene and ther mxture. Theory The theory of tme-resolved Four-Wave-Mxng (FWM) processes can be formulated based on thrd order perturbaton theory, whch s used to calculate the thrd order polarzaton, P (3) (t) responsble for the sgnal generaton (Mukamel, 1995; Tannor, 2007). In the semclasscal pcture presented here, the electromagnetc feld s treated classcally and the matter s descrbed quantum mechancally usng the wave functon approach. The fundamental observable n CARS s the thrd order polarsaton that s generated by the nteracton of three laser pulses namely the pump E pu ( pu, k pu ), Stokes E s ( s, k s ) and the probe E pr ( pr, k pr ) pulses. The tme dependent polarsaton P(t) nduced n a system wth a state vector (t) s gven by P()() t () t t (1) where s the dpole operator. Inorder to calculate the thrd order polarsaton,the perturbatve expanson of the wave functon s used. () N () t () t (2) N0 wave functons of dfferent orders can be obtaned teratvely as t H() tt N 1) N ()() () t e Hnt t t dt (3) Through successve nteractons wth the three pulses of the CARS process, the wave functon acqures spatal dependences assocated wth each of the pulses (Faeder et al., 2001). For hgher order

3 Femtosecond Coherent Ant-Stokes Raman Spectroscopy 527 wave functons, the order of the wave vectors denotes the tme orderng of the nteractons wth the felds. For example, the thrd order wave functon relevant for CARS s () (3) k pu k S k pr t whch s created by the absorpton of a pump photon wth wave vector k pu, stmulated emsson of a photon wth wave vector k s followed by the absorpton of a photon wth wave vector k pr. Ths sequence of events n CARS can be pctorally represented usng double-sded Feynman dagrams. The Feynman dagram relevant for CARS and the correspondng energy level dagram are shown n Fg. 1. The thrd order polarsaton responsble for the CARS sgnal generaton can be wrtten as (3)(0)(3) P ()() t () t. k pu ks k t c c pr (4) Ths equaton can be smply nterpreted as the evoluton of the ground state wave functon g (0) under the nfluence of the ground state Hamltonan H g tll the nteracton of the frst pump pulse E pu (t pu ) at tme t pu. The nteracton of the pump pulse creates an H t g pu excted wave functon E pu ()(0) t pu e g whch s nothng but the ground state wave functon scaled by [ E pu (t pu )]. Ths newly created excted wave packet then evolves under the excted state Hamltonan H e tll the nteracton wth the Stokes pulse E s (t s ) at tme t s. Ths nteracton causes a stmulated emsson to an excted vbratonal level g and the new wave functon generated by the stmulated emsson then evolves under the H g Hamltonan, tll the nteracton wth the probe pulse E pr (t pr )at tme t pr. The nteracton wth the probe pulse promotes the wave packet to the excted state, whch then evolves under the excted state Hamltonan H e tll the emsson of the ant-stokes feld. In order to calculate the ntensty of the ant- Stokes emsson, the thrd order polarsaton gven by equaton (4) can be converted to the frequency doman (Fourer transform) and used as the source term n the Maxwell s equaton. A Fg. 1: Energy level dagram (A) and double sded Feynman dagram (B) correspondng to CARS process B c 4 c ()() E P 2 2 (6) The soluton of ths equaton gves the sgnal ntensty as The thrd order wave functon () (3) k pu t k S k pr can be wrtten, assumng the energy level dagram gven n Fg. 1 as (3) () t 3 t t pr ts He () tt pr E pr () t pr e H g () t pr t S He () ts t pu H gt pu ES ()() ts e E pu t pu e (0) dt dt dt e g pu S pr (5) kl sn (3) ICARS I puis I prl kl 2 Where, 2 (7) (3) s the thrd order susceptblty, I ( = pu, S, pr) are the ntenstes of the three nput pulses and l s the nteracton length. k s the phase msmatch whch s gven by k = k pu k S + k pr k as (8) The phase msmatch s the wave vector msmatch between the three nput felds and the

4 528 Vnu V Namboodr et al. emtted sgnal. The snc functon dependence of the sgnal ntensty on the phase msmatch suggests that the sgnal rapdly decreases to zero wth ncreasng phase msmatch. Thus, for effcent CARS sgnal generaton proper phase matchng should be ensured. Ths s acheved by usng a specal beam geometry, whch s descrbed n the next secton. Expermental Setup for Tme-Resolved CARS Femtosecond laser pulses requred for the experment are generated by a commercal laser system (3 mj, 1 khz, Ampltude Technologes, Trdent-M) delverng pulses of duraton ~ 50 fs wth central wavelength 800 nm. Two multpass optcal parametrc amplfers (OPA; Lght Converson, TOPAS-C) are used n order to generate the dfferent wavelengths requred for the CARS experment.each of the OPA s pumped by 1 mj of 800 nm output from the Trdent-M system. One of the OPA output s contnuously tunable from 450 nm to nm and the other from 250 nm to 2700 nm. The pulse characterstcs are determned from autocorrelaton and FROG traces (Pulse Check, APE). The pump and probe pulses were of the same wavelength and derved from one of the OPA output usng a 50/50 beam spltter. The pump and Stokes wavelengths were 590 nm and 635 nm respectvely and correspond to a wavenumber dfference of 1200 cm 1. All the three laser pulse energes were fxed to be around nj usng neutral densty flters. The broad spectral wdth of the pulses enabled exctaton of Raman modes wthn a regon of +400 cm 1 from the centre of exctaton. For an effcent CARS sgnal generaton, perfect overlap of the nput pulses has to be ensured both spatally and temporally. A schematc of the complete expermental setup for CARS s shown n Fg. 2. Spatal overlap of the pulses s necessary to acheve phase matchng. The phase matchng s ensured by usng the Folded-BOXCAR arrangement of the nput beams. In ths arrangement, the three beams nvolved n the CARS process, pump, probe and Stokes are algned parallel to each other and are made to pass through three corners of a rectangular box wth the help of a mask as detaled n Fg. 3. The selecton of wavelength was based on the ntro group vbraton characterstcs. The three beams were then focused onto the sample usng a lens of 10 cm focal length. The tme delay between the pulses was adjusted usng computer controlled lnear translaton stages (Newport ILS 150) mounted wth retroreflectng mrrors. The pump and Stokes pulses for the CARS process were tme concdent and the tmng of the probe pulse was varable. The spatally separated CARS sgnal from the sample s collmated usng another lens (f=10 cm) and sent to a spectrometer (ANDOR Shamrock) attached wth a TE cooled CCD (ANDOR Newton). Neat lquds of ntromethane and ntrobenzene were Fg. 2: Expermental setup for CARS usng folded BOXCAR beam geometry: M-Mrror, BS- Beam Spltter, G- Gratng, and L-Lens. The delay stages are lnear translaton stages ftted wth a retroreflector

5 Femtosecond Coherent Ant-Stokes Raman Spectroscopy 529 Fg. 3: Folded BOXCARS beam arrangement Fg. 4: Tme and frequency resolved CARS spectrum of ntrobenzene used for the experments. Results and Dscusson In the followng, results from the CARS spectrum obtaned from neat lquds of ntromethane, ntrobenzene and ther mxture are dscussed. These molecules do not absorb n the vsble spectral regon, therefore the CARS process here does not nvolve any excted electronc state. CARS on Ntrobenzene The tme and frequency-resolved CARS transent of ntrobenzene s shown n Fg. 4. The transent shows well defned beatng structure arsng due to the smultaneous exctaton of dfferent vbratonal modes, whch fall wthn spectral wdth of the exctaton pulses. A beatng structure s observed n the transent CARS sgnal. Such beatng structures are well known and are due to quantum beatng of the dfferent coherently excted vbratonal modes of the molecules. To make these sgnal modulatons more vsble, Fg. 5A presents a frequency slce of the transent. An analyss of the beatng frequency can be performed by fast Fourer transformaton (FFT) and the frequency dfference between the beatng vbratonal modes s shown n Fg. 5B. The peak locaton along the FFT wavenumber axs corresponds to the wavenumber dfferences of two coherently excted vbratonal modes of ntrobenzene. It s now obvous that the complex oscllatory pattern of the tme doman sgnal s manly attrbutable to four peaks. An assgnment of the beatng wavenumbers s possble takng nto account the Raman modes excted by the femtosecond pump and Stokes pulses. It s now approprate to compare the obtaned results wth spontaneous Raman spectrum of ntrobenzene (Fg. 6). The beat frequences seen n the Fourer spectrum shows peaks at wavenumber postons 20 cm 1, 103 cm 1, 238 cm 1 and 337 cm 1. These peaks correspond to the frequency dfference between the 853 cm 1, 1003 cm 1, 1022 cm 1, 1107 cm 1 and 1347 cm 1 vbratonal modes of ntrobenzene. The hghest beat frequency of 337 cm 1 between the ntrobenzene vbratonal modes at 1003 cm 1 and 1347 cm 1 corresponds to a frequency of 10.1 THz. Ths s ndcatve of the hgh temporal resoluton of the experment, whch enables probng of terahertz phenomena wth apprecable accuracy. CARS on Ntromethane and Mxture Smlar experments were carred out n ntromethane and a mxture of ntromethane and ntrobenzene (1:1 v/v). The ntromethane CARS transent s domnated by a sngle Raman mode at 917 cm 1 and hence dd not show any beatng structure (Fg. 7B). In the ntromethane case, no FFT analyss was necessary as there was only one promnent Raman mode and

6 530 Vnu V Namboodr et al. A B Fg. 5: A: Tme-resolved CARS transent showng rapd modulatons (spectral slce). B: FFT correspondng to the transent showng the beat frequences from the beatng between the vbratonal modes of ntromethane (917 cm 1 ) and ntrobenzene (1003 cm 1 ). The presence of addtonal frequency components n the CARS transent can be used as a sgnature to dentfy components n a mxture. Further analyss s beng carred out to extract the dephasng tme of dfferent partcpatng vbratonal modes and to analyse how the dephasng tmes are nfluenced n a mxture. Conclusons Fg. 6: Spontaneous Raman spectrum of ntrobenzene (blue) and ntromethane (red) dsplayng the vbratonal modes excted by the femtosecond pump and Stokes pulses the CARS sgnal decays exponentally. The decay tme = T 2 /2 s 1139 fs (Shkurnov, 1994). The CARS transent of the mxture showed modulatons of the CARS sgnal (Fg. 7A) smlar to the case of ntrobenzene. Fourer analyss of the CARS transent of the mxture showed an addtonal beat frequency component at 86 cm 1 (Fg. 8) whch arses An expermental faclty for tme-resolved CARS usng femtosecond laser pulses has been descrbed wth two examples of organc ntro compounds. Spectrally dspersed CARS sgnal leads to a twodmensonal (tme and wavenumber) data set. An analyss of ths two-dmensonal tme doman data on the bass of Fourer transform methods enables one to obtan detaled spectral nformaton on the vbratonal mode structure n addton to the vbratonal dephasng tmes that are readly deducble from the decay behavour of the transent sgnals. It has been demonstrated that CARS spectroscopy wth femtosecond tme resoluton and wavenumber resolved detecton s a powerful tool for the characterzaton of dynamcs n the electronc ground state of molecules.

7 Femtosecond Coherent Ant-Stokes Raman Spectroscopy 531 A B Fg. 7: A: CARS transent from 1:1 (by volume) mxture of ntromethane and ntrobenzene. B: CARS transent from pure ntromethane Fg. 8: FFT of the CARS transent of mxture References Brown E, Zhang Q and Dantus M (1999) Femtosecond transentgratng technques: Populaton and coherence dynamcs nvolvng ground and excted states J of Chemcal Physcs Djaker N, Lenne P, Marguet D, Colonna A, Hadjur C and Rgneault H (2007) Coherent ant-stokes Raman scatterng mcroscopy (CARS): Instrumentaton and applcatons Nuclear Instruments and Methods n Physcs Research A El-Dasty F (2011) Coherent ant-stokes Raman scatterng: Spectroscopy and mcroscopy Vbratonal Spectroscopy Faeder J, Pnkas I, Knopp G, Pror Y and Tannor D J (2001) Vbratonal polarzaton beats n femtosecond coherent ant- Stokes Raman spectroscopy: A sgnature of dssocatve pump-dump-pump wave packet dynamcs J Chemcal Physcs Leonhardt R, Holzapfel W, Znth W and Kaser W (1987) Terahertz quantum beats n molecular lquds Chemcal Physcs Letters Monmayrant A, Weber S and Chatel B (2010) A newcomer s gude to ultrashort pulse shapng and characterzaton J Physcs B: Atomc Molecular and Optcal Physcs Mukamel S (1995) Prncples of Nonlnear Optcal Spectroscopy Oxford Unversty Press New York Potma E and Mukamel S (2013) Theory of Coherent Raman Scatterng. In: Coherent Raman Scatterng Mcroscopy (Eds: Cheng J and Xe X S) pp 3-42 CRC Press Taylor & Francs Group Roy S, Gord J R and Patnak A K (2010) Recent advances n coherent ant-stokes Raman scatterng spectroscopy: Fundamental developments and applcatons n reactng flows Progress n Energy and Combuston Scence

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