van den Broek, M.A.F.H.

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UvA-DARE is a service provided by the library of the University of Amsterdam (https://dare.uva.nl)UvA-DARE (Digital Academic Repository)Ultrafast redistribution of vibrational energy in liquidsvan den Broek, M.A.F.H.Publication date2002Link to publicationCitation for published version (APA):van den Broek, M. A. F. H. (2002). Ultrafast redistribution of vibrational energy in liquids.[Thesis, externally prepared, Universiteit van Amsterdam].General rightsIt is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s)and/or copyright holder(s), other than for strictly personal, individual use, unless the work is under an opencontent license (like Creative Commons).Disclaimer/Complaints regulationsIf you believe that digital publication of certain material infringes any of your rights or (privacy) interests, pleaselet the Library know, stating your reasons. In case of a legitimate complaint, the Library will make the materialinaccessible and/or remove it from the website. Please Ask the Library: https://uba.uva.nl/en/contact, or a letterto: Library of the University of Amsterdam, Secretariat, Singel 425, 1012 WP Amsterdam, The Netherlands. Youwill be contacted as soon as possible.Download date:11 Mar 2023SUMMARY Y Thiss thesis is devoted to the investigation of the dynamics of excited vibrations of smal moleculess and complexes in liquid solution and in the pure liquid. When excited vibra-tionall  states decay, energy is transferred to other modes, eventually leading to an equili-brationn over all degrees of freedom. The lifetime T, of the excited state, which is usually onn the order of picoseconds in the condensed phase, depends on both the nature and the availabilityy of the accepting modes. By looking at the lifetimes and vibrational dynamics of excitedd vibrations, a lot of information can be gathered on the couplings that exist within andd between molecules. Thee lifetimes and vibrational dynamics of several excited molecular vibrations are in thiss thesis investigated with vibrational pump-probe spectroscopy. In a pump-probe ex-periment,, a pump pulse excites a vibration of a significant fraction of the molecules in a sample.. This excitation causes a decrease in the absorbance of the sample, as there are less moleculess in the vibrational ground state that can absorb radiation at the v — o —> v = i transitionn frequency. The transmittance change is measured with a weaker probe pulse that passess through the sample after a variable delay, so that the evolution of the excited vibra-tionn can be followed in time. This form of time-resolved spectroscopy requires the light pulsess to be shorter than the typical time scale of the decay of the excitation. Thee transition frequencies of the molecular vibrations that were studied lie in the mid-infrared,, corresponding to wavelengths of I - I O yum, so short mid-infrared (laser) pulses aree needed. There are few lasers that emit in this wavelength region, therefore in most experimentss non-linear optical processes like difference-frequency generation (DFG) and opticall  parametric generation and amplification (OPG/OPA) are used to generate "ultrashort' mid-infraredd pulses from the output of lasers that emit in the visible or near-infrared. TwoTwo different hydrogen-bonded acid-base complexes were investigated. A relatively weakk hydrogen bond was encountered in complexes of hydrogen chloride (HC1) with di-ethyll  ether [ (CH,CH: ) 20 ]. The lifetime of the excited H-Cl stretch vibration was deter-minedd to be 0.9  0.2 ps. The relaxation is found to occur via an intermediate state with thee low-frequency (CH^CHJ.O  -H-Cl hydrogen bond as the main accepting mode of thee vibrational energy. The excited population of this hydrogen-bond mode decays in a sce-condd relaxation proces with a time constant of 3.1 5 ps. In hydrogen-bonded complexes off  the acid hydrogen fluoride (HH) with amines, the hydrogen-bonding between the acid HFF and the amine base is very strong. We studied the vibrational dynamics of the HF/FHF stretchh combination absorption band of complexes of HF with the weak organic base pyri-dinee in diluted pyridine solution. The relaxation of this excited vibration was also observed too occur in two steps. In the first step, energy is transferred from the excited combination vibrationn to the F-H-  -F hydrogen bond modes with a time constant of o.51 9 ps. In a secondd step, with a time constant of 2.6 3 ps, energy is transferred from the F-F hydro-genn bond modes to lower-frequency modes, leading to a local thermalisation of the energy. Althoughh the hydrogen-bond strengths are very different in both acid-base complexes, in 9i i 92 2 SUMMARY Y bothh cases a similar relaxation mechanism is seen, where the hvdrogen-bond mode is the mainn accepting mode in the first step of the relaxation. Thee relaxation of an excited molecular vibration can be an /'«tazmolecular process, in whichh energy is transferred to other vibrations in the same molecule, through anharmonic couplingss with the vibrational mode that was originally excited, or an />/?mnolecular pro-cess,, in which energy is transferred to vibrations of neighbouring (solvent) molecules or to low-frequencyy (solvent) modes. Two-colour pump-probe experiments on the C-H stretch vibrationn of pure bromoform (CHBr,) show that, compared to solutions of bromoform, thee reduction in the number of accepting modes provided by the solvent for intermolecular energyy transfer, greatly influences the mechanism and time scale of vibrational relaxation. Thee relaxation of the excited C-H stretch vibration in pure CHBr, occurs with a time constantt of 43  i ps via an intermediate level, which causes a transient redshifr of the C -HH stretch frequency. We identified the intermediate level with the singly excited C-H bendd mode. We observed that relaxation from this intermediate level is exceptionally slow andd non-exponential, which suggests the presence of a "bottleneck" in the relaxation, in contrastt to pure chloroform (CHC1,) and solutions of bromoform. The population of this bottleneckk state is very long-lived, which frustrates thermalisation. Thee influence of the number of vibrational modes in a molecule on intramolecular vibrationall  relaxation, is manifested in the investigation of the vibrational dynamics after excitationn of the C-O stretch vibration of the alcohols methanol and ethanol in carbon tetrachloridee (CCl4) solution. The relaxation of the excited C-O stretch vibration was observedd to occur in two steps. In the first step energy is transferred from the v — 1 state off  the C-O stretch vibration to an intermediate state with a time constant of 3.2  0.2 ps forr methanol and 3.2  0.7 ps for ethanol. The intermediate state is most likely formed by thee n — 3 or 4 state of the C - O -H bending mode, which is also known as the torsional mode.. In the second step energy is transferred from this intermediate state to low-energy modes,, leading to a full equilibration of the energy. In methanol this thermalisation occurs withh a time constant of 28  1 ps. In ethanol the second step is faster, with a time constant off  12  2 ps, because of the higher number of available accepting modes. Inn the last chapter of this thesis, we focus on the non-linear optical proces that is con-venientlyy used to generate the mid-infrared pulses for the experiments featured in the other chapters.. The focus is on the correlation properties of parametrically generated light. These propertiess arise from the fact that OPG is a very special type of conversion process, where-onee starts with an electromagnetic wave at one frequency and ends up with waves at three frequencies.. This proces can only be understood using a quantum mechanical description off  the electromagnetic field. The parametrically generated light is the macroscopic manifes-tationn of the quantum fluctuations of the zero-photon electromagnetic field that seed the parametricc generation process. The temporal coherence or correlation time r t of paramet-ricallyy generated mid-infrared light is determined by measuring the twin-correlation peak inn the sum-frequency spectrum as a function of delay between the signal and idler. The cor-relationn time rL of the generated signal and idler fields was found to lie in the picosecond rangee and is related to the bandwidth Aw by rL = i/Aw , 

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Ultrafast redistribution of vibrational energy in liquids

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Ultrafast redistribution of vibrational energy in liquids

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