Upconversion Methods

Solvation dynamics are measured using the more reliable energy relaxation method after a local perturbation [83-85], typically using a femtosecond-resolved fluorescence technique. Experimentally, the wavelength-resolved transients are obtained using the fluorescence upconversion method [85], The observed fluorescence dynamics, decay at the blue side and rise at the red side (Fig. 3a), reflecting typical solvation processes. The molecular mechanism is schematically shown in Fig. 5. Typically, by following the standard procedures [35], we can construct the femtosecond-resolved emission spectra (FRES, Stokes shifts with time) and then the correlation function (solvent response curve) ... 

In this paper we describe a novel apparatus that combines the "upconversion" method with multichannel optical detection. This instrument records, for each laser pulse, a high-resolution emission spectrum. at a chosen delay time, i.e. I(t=constant, X). 

The sample fluorescence is time resolved by the upconversion method. The fluorescence is upconverted in a Type II KDP crystal by a portion of the 1064 nm light which travels a fixed path that is portrayed in Figure 1. The "upconverted" light intensity... 

We employ method B to study effects of this type. In this mode, our apparatus yields relative high-resolution fluorescence spectra at different time windows after excitation of the sample by the 355 nm pulse. The spectra are acquired by the upconversion method. The upconverted fluorescence spectrum is recorded simultaneously at all monitored wavelengths by an optical multichannel analyzer. It is constructed from a poly-chromator (HR320 Instruments SA) and an intensified silicon photodiode array detector (Princeton Applied Research Model 1412). The detector is interfaced to our Cromemco computer. 

Wavelength Sensitivity. The overall wavelength sensitivity of emission spectrometers plays a central role in determining the accuracy of the recorded data and the range of emission wavelengths that can effectively be studied by a particular apparatus. The upconversion method adds a source of wavelength dependent sensitivity that is both useful and problematic. 

Direct experimental measurement of the lifetimes of the 83 states (153) of the porphyrins and metalloporphyrins requires the use of ps or fs laser techniques transient pump-probe and fluorescence upconversion methods have been most... 

Soukka T, Rantanen T, Kuningas K (2008) Photon upconversion in homogeneous fluorescence-based bioanalytical assays. Ann N Y Acad Sci 1130 188-200, Fluorescence Methods and Applications Spectroscopy, Imaging, and Probes... 

Two-color excitation can thus provide orders of magnitude intensity gains for GSA/ESA upconversion processes through the optimization of both Oq and o. Eor practical applications such as lasing or imaging, this can lead to superior performance [6]. Two-color upconversion excitation methods have been applied in other cases to generate efficient upconversion where little or none was observed under one-color conditions, or to advance mechanistic studies. 

Figure 3. Time-resolvedfluorescence traces of 3 HF in MeOH and MeOD acquired by Method A (see text). The 413-nm emission was indirectly monitored by upconversion at 297 nm, and the543-nm emission was monitored by upconversion at 360 nm. The points in the figure are experimental data, and the solid lines are computer fits to this data. (The fitting algorithm has been described elsewhere (1).J...

The methods discussed so far, fluorescence upconversion, the various pump-probe spectroscopies, and the polarized variations for the measurement of anisotropy, are essentially conventional spectroscopies adapted to the femtosecond regime. At the simplest level of interpretation, the information content of these conventional time-resolved methods pertains to populations in resonantly prepared or probed states. As applied to chemical kinetics, for most slow reactions (on the ten picosecond and longer time scales), populations adequately specify the position of the reaction coordinate intermediates and products show up as time-delayed spectral entities, and assignment of the transient spectra to chemical structures follows, in most cases, the same principles used in spectroscopic experiments performed with continuous wave or nanosecond pulsed lasers. 

In fluid solvents at room temperature, spectral relaxation is usually comjdete prior to emission and occurs within abont 10 ps. This process is too rapid to be resolved with the usual instrumentation for TD or FD fluorescence. However, advances in laser technology and methods for ultrafast spectroscopy have resulted in an increasing interest in picosecond and femtosecond solvent dynaaiics. Becmise of the rapid timescale, the data on solvent dynamics are usually obtained using flurvescence upconversion. Hiis method is described in Section 4.7.C. lypical data are... 

Pedroni, M., PiccineUi, F., Passuello, T., Gioarola, M., Mariotto, G., Polizzi, S., et al., 2011. Lanthanide doped upconverting colloidal CaF2 nanoparticles prepared by a single-step hydrothermal method toward efficient materials with near infrared-to-near infrared upconversion emission. Nanoscale 3, 1456—1460. 

As is well known photochemical upconversion has been demonstrated to be one of the most promising methods for best exploiting solar light. Schmidt and Castellano have elaborated on the parameters that affect... 

Kuisheng Y, Yan L, Chaoyi Y, Liping L, Chanhua Y, Xiyan Z. Upconversion luminescence properties of Ho " ", Tm, Yb co-doped nanocrystal NaYF4 synthesized by hydrothermal method. J Rare Earths 2006 24(6) 757. 

Frequency upconversion of 800 nm ultrashort 175 fs optical pulses by two-photon absorption in a stilbenoid compound-doped polymer (PMMA) optical fiber was reported [28]. By the intensity-dependent transmission method, the two-photon absorption cross section was deduced. The combination of a well-designed organic chromophore incorporated into a fiber geometry is appealing for the development of an upconversion blue polymer laser. Upconversion fluorescence and optical power limiting effects based on the two- and three-photon absorption process of a frans-4,4 bis(pyrrolidinyl)stilbene were investigated [29]. The molecular TPA cross section three-photon absorption (3PA) cross section g3 at 720-1000 nm were measured. The 3PA-induced optical power-limiting properties were also illustrated at 980 nm. 

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