Instrumentation pump-probe

Although very detailed, fundamental information is available from ultrafast TRIR methods, significant expertise in femtosecond/picosecond spectroscopy is required to conduct such experiments. TRIR spectroscopy on the nanosecond or slower timescale is a more straightforward experiment. Here, mainly two alternatives exist step-scan FTIR spectroscopy and conventional pump-probe dispersive TRIR spectroscopy, each with their own strengths and weaknesses. Commercial instruments for each of these approaches are currently available. 

Figure 20.2. Schematic outline of typical pump-probe-detect experiments with femtosecond pulses, a molecular beam source, and mass spectrometric detection of transient species. Computer control and data processing instruments, as well as various optical components, are not shown. The time separation Af between pump and probe pulses is dictated by the difference in optical path lengths. Ad, traversed by the two components of the original pulse.

Ellington et al.40) used femtosecond pump-probe spectroscopy to probe directly the arrival of electrons injected into the TiOz film with near- and mid-IR that probe the absorption at 1.52 jum and in the range of 4.1-7.0 jUm. Their measurements indicate an instrument limited 50 fsec upper limit on the electron injection time. These observations suggest that electron injection from Dye 2 to... 

Time-resolved spectroscopy is performed using a pump-probe method in which a short-pulsed laser is used to initiate a T-jump and a mid-IR probe laser is used to monitor the transient IR absorbance in the sample. A schematic of the entire instrument is shown in Fig. 17.4. For clarity, only key components are shown. In the description that follows, only those components will be described. A continuous-wave (CW) lead-salt (PbSe) diode laser (output power <1 mW) tuned to a specific vibrational mode of the RNA molecule probes the transient absorbance of the sample. The linewidth of the probe laser is quite narrow (<0.5 cm-1) and sets the spectral resolution of the time-resolved experiments. The divergent output of the diode laser is collected and collimated by a gold coated off-axis... 

Figure 11-5. Pump-probe transient ionization signal of 1,3-DMU in the gas phase with the pump and probe wavelengths at 251 and 220 nm, respectively. Hollow circles represent experimental data, and the solid line is a theoretical fit including a single exponential decay convoluted with the instrumental response (dashed trace). The exponential decay constant is 52 ns, while the full width at half maximum of the Gaussian function is 5.5 ns...

In this experiment type, the detector of the transmitted probe light behaves as a photon integrator only and the time resolution derives from the instrument s capability for measuring the difference in the time of arrival of the pump and probe pulses. This capability provides an intrinsic limitation to the pump-probe method, which depends critically on the width of the pulses being employed. As indicated earlier, exceedingly short duration pulses are commonplace these days. 

Abstract. We present a novel instrument combining femtosecond pump-probe spectroscopy with broadband detection and confocal microscopy. The system has 200-fs temporal resolution and 300-nm spatial resolution. We apply the instrument to map excited state dynamics in thin films of polyfluorene-polymethylethacrylate blends. 

Figure 5. Set of time-resolved UV-near-IR spectroscopic data (3.44-0.99 eV) following the femtosecond UV excitation of an aqueous sodium chloride solution ([H20]/[NaCl] = 55). An instrumental response of the pump-probe configuration at 1.77 eV (n-heptane) is also shown in the middle part of the figure. The ultra-short-lived components discriminated by UV and IR spectroscopy correspond to low or high excited CTTS states (CTTS, CTTS ), electron-atom pairs (Che pairs), and excited hydrated electrons (ehyd )- The spectral signature of relaxed electronic states (ground state of a hydrated electron, (ehyd) electron-cation pairs, a e hyd) observed in the red spectral region.

Experimental Setup. An obvious extension of the one-color pump-probe experiments is the application of two-color experiments in which two independently tunable dye lasers share the same pump laser. One can use the same high repetition rate and obtain spectral evolutions on excitation at selected wavelengths. The measurements are performed in essentially the same way as one-color experiments.A disadvantage is the broadened instrument function (cross-correlation function) caused by time jitter between the two pulses, since they are not obtained from the same dye laser. This leads to a full-width half-maximum (fwhm) value of the instrument function of approximately 5-10 psec. 

Fig. 7. Diagram of a multiplexed SPT instrument using a multianode MCP detector. The signals from several channels (representing different wavelengths) can be measuretd in parallel. In the excitation beam the possibility of a pump-probe arrangement using two laser pulses for measuring the fluorescence kinetics of short-lived intermediates is indicated (see text).

The ILIT program at Brookhaven National Laboratory has been terminated. We have demonstrated that our best instrumentation can now attain nanosecond and possibly subnanosecond time resolution (Sec. IV.E Fig. 6). The holy grail for those who study ultrafast interfacial kinetics remains the development of a pump-probe technique whose time resolution would be limited only by the operative physical chemical processes (see Sec. V.E). We did not achieve that goal, but ILIT could be a component in... 

Fig. 6.8 Femtosecond pump-probe spectroscopy of diiodoBODIPY (10). (a) Inverted absorption spectrum (6) and transient absorption spectra at 0.5 (1), 20 (2), 50 (3), 110 (4), and 350 (5) ps delay between pump and probe pulses, (b) Differential absorption spectra of intermediate states obtained after modeling the experimental data by a three-exponential equation (see text) that is convoluted with the instrumental response function (1) zero delay (sum of all amplitudes, S2 state), (2) ultrafast relaxation of the S2 state (sum of A2, A3, A4 amplitudes), (3) thermalized Sj state (sum of A3 and A4 amplitudes), (4) Tj state (A4 amplitude). Reprinted with permission from [24]...

The high costs associated with specialist ultrafast laser techniques can make their purchase prohibitive to many university research laboratories. However, centralised national and international research infrastructures hosting a variety of large scale sophisticated laser facilities are available to researchers. In Europe access to these facilities is currently obtained either via successful application to Laser Lab Europe (a European Union Research Initiative) [35] or directly to the research facility. Calls for proposals are launched at least annually and instrument time is allocated to the research on the basis of peer-reviewed evaluation of the proposal. Each facility hosts a variety of exotic techniques, enabling photoactive systems to be probed across a variety of timescales in different dimensions. For example, the STFC Central Laser Facility at the Rutherford Appleton Laboratory (UK) is home to optical tweezers, femtosecond pump-probe spectroscopy, time-resolved stimulated and resonance Raman spectroscopy, time-resolved linear and non-linear infrared transient spectroscopy, to name just a few techniques [36]. 

In on-line control, an automated sampling system attached to a reactor or by-pass system is used to extract the sample, if needed conditioned, and presented to an analytical instrument or probe for measurement. Sampling delays can be significant in on-line installations, because of the transit line and gear pumps. On the other hand, on-Une devices are isolated from the main stream by the use of a gear pump, and the temperature and pressure of the polymer sampling flow can thus be controlled. Their maintenance can therefore be done without a complete process shutdown. Many of the apparent disagreements between results from split side stream on-line analysers and results from the laboratory can be traced back to differences which occur because samples differ in acquisition time, location and stability. The main characteristics of on-line process analysis are summarised in Table 7.4. 

When solution must be pumped, consideration should be given to use of holding tanks between the dry feed system and feed pumps, and the solution water supply should be controlled to prevent excessive dilution. The dry feeders may be started and stopped by tank level probes. Variable-control metering pumps can then transfer the alum stock solution to the point of application without further dilution. Means should be provided for calibration of the chemical feeders. Volumetric feeders may be mounted on platform scales. Belt feeders should include a sample chute and box to catch samples for checking actual delivery with set delivery. Gravimetric feeders are usually furnished with totalizers only. Remote instrumentation is frequently used with gravimetric equipment, but seldom used with volumetric equipment. 

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