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Noncollinear phase matching in fluorescence upconversion.
Rhee, Hanju,Joo, Taiha Optical Society of America 2005 Optics letters Vol.30 No.1
<P>We investigate noncollinear sum-frequency generation in a time-resolved fluorescence upconversion experiment to eliminate group-velocity mismatch (GVM), a major mechanism that deteriorates time resolution. The noncollinear geometry inherently causes phase-front mismatch (PFM) that also spoils time resolution. The effects of GVM and PFM on the time resolution are studied by numerical calculations and experiments. Based on the investigation, a fluorescence upconversion apparatus with time resolution better than 45 fs (FWHM) is demonstrated.</P>
Lee, Junghwa,Joo, Taiha Korean Chemical Society 2014 Bulletin of the Korean Chemical Society Vol.35 No.3
Photophysics of 10-hydroxybenzo[h]quinoline (HBQ) has been in controversy, in particular, on the nature of the electronic states before and after the excited state intramolecular proton transfer (ESIPT), even though the dynamics and mechanism of the ESIPT have been well established. We report highly time resolved fluorescence spectra over the full emission frequency regions of the enol and keto isomers and the anisotropy in time domain to determine the accurate rates of the population decay, spectral relaxation and anisotropy decay of the keto isomer. We have shown that the ~300 fs component observed frequently in ESIPT dynamics arises from the $S_2{\rightarrow}S_1$ internal conversion in the reaction product keto isomer and that the ESIPT occurs from the enol isomer in $S_1$ state to the keto isomer in $S_2$ state.
Ultrafast time-resolved fluorescence by two photon absorption excitation
Kim, Chul Hoon,Joo, Taiha The Optical Society 2008 Optics express Vol.16 No.25
<P>We report a time-resolved fluorescence apparatus utilizing fluorescence upconversion by noncollinear sum frequency generation and two photon absorption as an excitation. Near perfect time-resolution is achieved with 20 fs pulses to give the instrument response of 33 fs (FWHM) over the entire fluorescence wavelength for a 100 microm thick mixing crystal. Through experiments and numerical simulations, it is shown that 40 fs time-resolution can be obtained even for a 580 microm thick mixing crystal at a fluorescence wavelength longer than 500 nm.</P>
Excited State Intramolecular Proton Transfer Dynamics of 1-Hydroxy-2-acetonaphthone
Kim, Jinyong,Heo, Wooseok,Joo, Taiha American Chemical Society 2015 The Journal of physical chemistry B Vol.119 No.6
<P>Excited state intramolecular proton transfer (ESIPT) of 1-hydroxy-2-acetonaphthone (HAN) has been in controversy, mainly because its Stokes shift is small compared to those of typical ESIPT molecules. We have investigated excited state dynamics of HAN by time-resolved fluorescence with a resolution high enough to record the nuclear wave packet motions in the excited state. Population dynamics of both the normal and tautomer forms were recorded together with the wave packet motions of the tautomer in the excited state, which confirm the ESIPT of HAN. The population dynamics of the normal and tautomer forms imply that the ESIPT dynamics is biphasic with two time constants <25 and 80 fs. Theoretical analysis of the vibrational modes of the tautomer excited impulsively reveals that major part of the change for the ESIPT reaction is on the naphthalene ring.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/jpcbfk/2015/jpcbfk.2015.119.issue-6/jp5088306/production/images/medium/jp-2014-088306_0006.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/jp5088306'>ACS Electronic Supporting Info</A></P>
Near-infrared cavity-dumped femtosecond optical parametric oscillator.
Min, Chang-Ki,Joo, Taiha Optical Society of America 2005 Optics letters Vol.30 No.14
<P>We demonstrate a high-energy near-infrared cavity-dumped femtosecond optical parametric oscillator (OPO) based on periodically poled lithium niobate. The laser generates 90 nJ pulses at a repetition rate of up to 1 MHz when synchronously pumped by 800 mW output from a femtosecond Ti:sapphire laser. The laser is broadly tunable from 1.0 to 1.5 microm in the signal branch, with a pulse duration of < 60 fs at 1.2 microm. High intracavity power is achieved by running the laser in the regime of positive group-velocity dispersion.</P>
Coherent excited state intramolecular proton transfer probed by time-resolved fluorescence
Kim, Chul Hoon,Joo, Taiha Royal Society of Chemistry 2009 Physical chemistry chemical physics Vol.11 No.44
<P>An ultra-fast chemical reaction can act as an impulsive excitation of the vibrations. Excited state intramolecular proton transfer in 10-hydroxybenzo[<I>h</I>]quinoline proceeds in 13 fs. As a result, product vibrational modes with their periods longer than (13 × 2) fs are coherently excited in the product potential surface, which can be observed most unambiguously by the oscillations in the time-resolved fluorescence signal of the product keto isomer.</P> <P>Graphic Abstract</P><P>An ultra-fast chemical reaction can act as an impulsive excitation of the vibrations, which was directly observed by time-resolved fluorescence. <IMG SRC='http://pubs.rsc.org/services/images/RSCpubs.ePlatform.Service.FreeContent.ImageService.svc/ImageService/image/GA?id=b915768a'> </P>
Active Role of Proton in Excited State Intramolecular Proton Transfer Reaction
Lee, Junghwa,Kim, Chul Hoon,Joo, Taiha American Chemical Society 2013 The journal of physical chemistry. A, Molecules, s Vol.117 No.7
<P>Proton transfer is one of the most important elementary reactions in chemistry and biology. The role of proton in the course of proton transfer, whether it is active or passive, has been the subject of intense investigations. Here we demonstrate the active role of proton in the excited state intramolecular proton transfer (ESIPT) of 10-hydroxybenzo[<I>h</I>]quinoline (HBQ). The ESIPT of HBQ proceeds in 12 ± 6 fs, and the rate is slowed down to 25 ± 5 fs for DBQ where the reactive hydrogen is replaced by deuterium. The results are consistent with the ballistic proton wave packet transfer within the experimental uncertainty. This ultrafast proton transfer leads to the coherent excitation of the vibrational modes of the product state. In contrast, ESIPT of 2-(2′-hydroxyphenyl)benzothiazole (HBT) is much slower at 62 fs and shows no isotope dependence implying complete passive role of the proton.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/jpcafh/2013/jpcafh.2013.117.issue-7/jp311884b/production/images/medium/jp-2012-11884b_0002.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/jp311884b'>ACS Electronic Supporting Info</A></P>
Park, Myeongkee,Kim, Chul Hoon,Joo, Taiha American Chemical Society 2013 The journal of physical chemistry. A, Molecules, s Vol.117 No.2
<P>Intramolecular charge transfer (ICT) of DMABN has been the subject of extensive investigations. Through the measurements of highly time-resolved fluorescence spectra (TRFS) over the whole emission region, we have examined the ICT dynamics of DMABN in acetonitrile free from the solvation dynamics and vibronic relaxation. The ICT dynamics was found to be characterized by a broad range of time scales; nearly instantaneous (<30 fs), 160 fs, and 3.3 ps. TRFS revealed that an ICT state with partially twisted geometry, ICT(P), is formed within a few hundred femtoseconds either directly from the initial photoexcited state or via the locally excited (LE) state. The ICT(P) state undergoes further relaxation along the intramolecular nuclear coordinate to reach the twisted ICT (TICT) state with the time constant of 4.8 ps. A conformational diversity along the rotation of the dimethylamino group was speculated to account for the observed diffusive dynamics.</P>