Implementation of Laser-Based Nanosecond Luminescence and Pump-Probe
Experiments in the Undergraduate Physical Chemistry Laboratory
(Based on poster presentation: ACS National Conference, San Diego, April '01)
Roland D. Saito, Rama Viswanathan* (PI), Department of Chemistry, Beloit College, Beloit, WI 53511
Introduction
The use of pulsed-laser systems has become important in modern physical chemistry. By designing simple laser systems for pulsed excitation experiments, students can gain knowledge and experience in modern physics, photochemistry, and laser-based technology.
During the period June - August, 2000, we were able to set-up a pulsed-laser luminescence system at Beloit College, partially funded by NSF grant DUE-9952604. A Q-switched YAG laser with UV harmonics at 355 nm and 266 nm was used for excitation. Data were acquired using a fast photodetector with nanosecond time resolution, coupled to a 1-Ghz analog bandwidth digital oscilloscope. We are currently implementing a novel pump-probe setup using an optical delay line whose length will be varied under computer control by mounting a retroreflector on a toy train engine running on a track [1].
The fluorescence of a number of different molecules, such a quinine bisulfate (QBS), degassed 1,2:3,4 dibenz-anthracene (DBA), anthracene, naphthalene, and terbium (III) complexes in solution at room temperature were obtained. Here, we present results for the time-resolved fluorescence of quinine bisulfate quenched by chloride ions and compare the technique and results to the traditional steady-state spectrofluorophotometer experiment.
Figure 1: The picture above shows the setup of the pulsed experiments. The laser beam passes through the iris for beam shaping and the UV beam is reflected off the filter and passes through the cell. Because the laser pulse contains both IR (1064 nm) and green light (532 nm) a green filter was needed.
Equipment Used For Pulsed Experiments
Continuum Minilite II Nanosecond Pulsed Nd:YAG Laser With UV Harmonics
Agilent Infiniium 1-GHz Analog Bandwidth, 4GSamples/sec Digital Oscilloscope Model 54835A
Green Reflective Filter (Oriel 54784)
New Focus Photodetector With Nanosecond Time- Resolution Model 1621
ESCO GG-400 UV Filter
Figure 2: Agilent Infiniium 54835A Digital Oscilloscope
Experiment: Time-Resolved Fluorescence of Quinine Bisulfate (QBS) Quenched by Chloride Ions
Using the setup shown in Figure 1 above, the lifetimes of the blue fluorescence (max emission at 456 nm) of 2.0x10–4 M QBS in 0.005M H2SO were obtained as a function of chloride ion concentration (0 to 0.005M).
Figure 3: Oscilloscope traces corresponding to nanosecond time-resolved fluorescence decay of QBS quenched by various concentrations of chloride ions. Analysis of the decay with the longest lifetime yields a lifetime 0 = 18.84 ns for unquenched QBS.
Figure 4: The Stern-Volmer plot given by the equation o/ = 1+kqo[Cl] is shown above. As expected, the plot of the o/ vs [Cl] was linear and the value of kq calculated (6.1(2) x 109 M-1s-1) is in moderate agreement with the values given in the literature [2,3].
Figure 5 (below) : Blue fluorescence of Quinine Bisulfate
Deconvolution
For luminescence with short lifetimes comparable to the temporal duration of the laser pulse (5 ns fwhm), deconvolution of the laser pulse from the decay profile is necessary in order to obtain accurate lifetimes. This has been described in detail by Demas [4].
A version of the algorithm described by Demas [4] was easily implemented into Microsoft Excel as a Visual Basic for Applications (VBA) script, and the laser pulse profile was deconvoluted from the raw data stored in an Excel spreadsheet.
Steady State Experiments
The traditional steady state experiments [5] were run using a Shimadzu RF-5000 spectrofluorophotometer. Using the fluorimeter, fluorescence intensities were determined for quinine bisulfate quenched with various concentration of chloride ions. The Stern-Volmer equation [6] is Io/I = 1+kqo[Cl], where I is the fluorescence intensity. As expected, the plot of Io/I vs [Cl] was linear. Using o (18.84 ns) from the pulsed experiment, kq is calculated to be 6.5(2) x 109 M-1s-1.
Comparing the Steady-State and Lifetime Experiments
Using the Stern-Volmer plot, students can compare calculations done from both steady state and lifetime experiments. The relationship between the fluorescence intensities and lifetimes are given by
Io/I = o/= 1+kqo[Cl].
If both steady state and lifetime (pulsed) experiments give valid data, the slopes of both Stern-Volmer plots should yield identical slopes (kqo). Based on our preliminary set of experiments, the value of kq calculated from the steady-state data (6.5(2) x 109 M-1s-1) is different from the value (6.1(2) x 109 M-1s-1) calculated from the pulsed experiments. However, our preliminary analysis of the intensity data in the steady-state experiments did not include corrections for baseline drift and wavelength dependence of quantum yields.
Pump-Probe Experiments in Progress
Figure 6 : Diagram of pump-probe setup
The excited singlet state of a molecule is produced by a pump laser pulse (2) (355 nm Nd:YAG Laser (1) third harmonic), which also pumps a dye laser (3) (VSL DYE) to produce a delayed probe beam (4) tuned to wavelength corresponding to excited triplet to triplet absorption. Two photodetectors are needed to monitor intensities of both the incident (5) and transmitted (6) probe pulse.
The probe beam (4) is delayed on an optical delay line (7). Currently, the delay distances and corresponding times are varied by manually shifting mirrors. We soon plan to automate the delay variation by using a setup which consists of a computer-controlled toy train engine equipped with a retroreflector running on a track (7, 8) [1]. Time delays between 6 ns and 30 ns can be obtained, limited by the distance constraints in our laboratory.
We are currently working on an experiment to observe excited triplet-triplet transient absorption in dibenzanthracene with a probe laser pulse at 440 nm (Coumarin 120 laser dye) following excitation of the singlet state at 355 nm (pump pulse). Yee and Kliger [7] have reported that roughly equal concentrations of excited singlet and triplet are present at a 25 ns time delay after excitation, making this experiment feasible in terms of delay times that are accessible in our laboratory.
Acknowledgements
This research was conducted with funding provided by NSF grant DUE-9952604 and Beloit College.
References
1.Hui-Rong, X., and Benson, S.V. Laser Focus, Mar. 1981, pp. 54-58.
2.Chen, R. Analytical Biochemistry 1974, 57, 593.
3.Barrow, D. A., and Lentz, B.R. Chem. Phys. Lett. 1984, 104, 163.
4.Demas, J.N. Excited State Lifetime Measurements, Academic Press 1983,
pp 256-259.
5.Bigger, S., Ghiggino, K., Meilak, G., and Verity, B. J. Chem. Ed. 1992, 69, 675.
6.Pillings, M., and Seakins, P. Reaction Kinetics, Oxford Science Publications
1995, pp. 144-154.
7. Yee, G.G. and Kliger, D.S., J. Phys. Chem.1983, 87, 1887.