Time-resolved spectroscopy

Investigation of the phenomena that take place in excited states requires the ability to follow some measurable physical properties of the system from its excitation to its relaxation to the ground state. The time scale of these processes varies from microseconds to a few picoseconds. Today, there are different techniques that can be employed in the study o f such fast phenomena. The most exploited techniques involve the measurement of either absorbance or emission from a sample that is excited by a short laser pulse (pulse time has to be shorter than the excited state lifetime). Many variations are possible, depending on the time-scale o f the process under investigation.

The simplest time-resolved technique is based on transient absorption measurements. Here, a strong laser pulse is used to excite a portion of the sample. Perpendicular to the laser is a Xe arc lamp, which produces the analyzing light whose transmittance is recorded at different times after the pulsed excitation; the instrumentation is synchronized by a trigger system, which connects the laser source to the receiver/analyzer (Figure 4- 1 2). Absorption (at one selected wavelength) is recorded at different times after irradiation and the resulting A(t) provides information about lifetimes of the excited states. It is also possible to obtain spectra of the excited states but because of the shortness of the time available for each acquisition, a very long time (thousands of acquisitions) is required to obtain accurate data and the power of the laser can cause decomposition of the compounds under examination. In order to increase the amount of light that can be collected by the receiver/analyzer system, a few modifications to the base instrumentation are required. The continuous source is

modified by a short time high voltage application; in this way the intensity of the source can be increased up to 30 times for 1 ms. Similarly, the use of photomultipliers between the monochromator and the detector can result in more accurate data.

NdIY AG laser

!

Trigger �pl'

�

_{receiver /} source analyzer monochromator

Figure 4-12 Schematic representation ofa ns time-resolved absorbance spectrometer

The faster the process under investigation, the more difficult it is to create short pulses and fast analysis techniques. For phenomena slower then l O ns, a Nd-YAG laser is typically used to excite the sample. This laser produces a very intense beam at l O64 run which can be transformed to its harmonics at 532, 355 and 266 nm, useful for UV­ vis spectroscopy. When investigations of faster processes (lifetime < 1 ns) are required, more complicated instrumentation has to be employed; in particular, the laser source has to be able to produce shorter pulses. Pulses of 1 8-35 ps can be achieved by modifying the NdIY AG laser source with a mode-locked, cavity dumped system. In this adaptation of UV -vis absorption spectroscopy (Figure 4- 1 3), the laser beam is split in two parts, which works as both the exciting and analyzing beam; this second beam is then transformed into 'white light ' by passing it through a cell containing D20ID3P04 and splitting it to two different portions of the sample. The exciting beam is delayed by a computer-controlled optical path and is directed to one o f the two portions irradiated with the analyzing light. Finally, the two analyzing beams are recorded by two sets of diode arrays which provide two sets o f A(t), from which the time-resolved spectra derive. Accurate data usually requires the average o f hundreds of acquisitions.

laser to white light converter beam splitter NdiY AG laser cavity dumped 20-35 ps pulse

optical delay line

receiver / analyzer

Figure 4-13 Schematic representation of a ps time-resolved absorbance spectrometer

Similar information about excited states can be obtained by emission spectroscopy. For processes in the nanosecond time domain, a modified Nd-Y AG laser source

(similar to the source described for absorption spectroscopy) is generally used to excite the sample. The other essential part of this kind of apparatus (Figure 4- 1 4) is a digital oscilloscope in the receiverlanalyzer apparatus, which is triggered by the laser pulse to correlate emission vs. time. While this method is very useful for qualitative data, it is not very accurate and the powerful NdlY AG laser source can cause decomposition of the sample.

NdIY AG laser _Trigger

Pockell cell 5 ns

!

receiver /

analyzer

The most accurate data on lifetimes can be obtained by time-correlated single photon counting (TCSPC). This technique utilizes the apparatus shown in Figure 4- 1 5, a less powerful N2 or Ar lamp is the source which provides I ns long pulses and a time-to­ amplitude converter (TAC) is the core of the instrument. The TAC creates an increasing � V between two electrodes when it is triggered by the first photon emitted by the lamp. By using a multichannel analyzer, in which every channel records at one specific � Vc, a correlation between time (from � V) and number of photons emitted can be obtained. This technique allows 1 000 measurement per second to be made and, therefore, data can be easily obtained as the average of millions of exponential decays producing very accurate measurements of lifetimes of excited states.

N2 or D2 laser 1 ns monochromator

�

TAC sample monochromator

Figure 4-15 Schematic representation of single photon counting apparatus

The investigation of emIssIons ID the picosecond time domain reqUITes more sophisticated equipment. An example of one of the instruments used in the course of this collaboration with researchers at IFOS-CNR is shown in Figure 4- 1 6. The laser source is the same as already described for picosecond absorption spectroscopy

(NdlYAG modified with a mode-locked, cavity dumped unit) and the trigger system and the optical delay path are also the same. The main modification in this apparatus is the use of a streak camera to record the time-resolved emissions. The emitted light is passed through a spectrograph in which a grating disperses this beam horizontally. The second step involves the use of the streak camera in which a photocathode transforms this light into electrons. Finally, the horizontal electron flow is deflected vertically by the triggered increasing � V and ends up exciting a phosphorous screen.

The result is a two-dimensional image where the axes represent the wavelength of the radiation and the delay from the excitation, and the intensity of the image is related to the intensity of the emission; this image is then elaborated by a computer. Accurate measurements usually require thousands of acquisitions and the resolution is around 20 ps.

optical delay

beam NdlY AG laser

sample cavity dumped

splitter _{20-35 ps pulse}

trigger

streak Image

spectrograph

camera analyzer

Figure 4-16 Schematic representation of a ps time-resolved emission spectrometer

In document Synthesis and properties of fully conjugated porphyrin arrays for light harvesting : a thesis presented in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Chemistry at Massey University, Palmerston North, New Zealand (Page 130-134)