Kenneth 1. Duchin, Yoon S. Lee,
I
ond James W. Mills1 Drew Univers~ty Madison, New Jersey 07940
Quenching of I, Vapor ~uorescence Excited with He-Ne Laser Light A kinetics-spectroscopy experiment
This paper describes an advanced undergraduate fluorescence auenchinn.. experiment usinn an inexpensive . He-Ye lasir for fluorescence excitation. The experiment promotes understanding of the following educational points (1) steady state kinetics of competing processes (2) hard sphere gas collision theory (3) rovibronic diatomic molecule structure and spectroscopy
(4) properlies of gas lasers, including spectral distribution and
cavity modes (5) vapor pressure as a function of temperature (6) compounding of experimental errors in a linear least
squares data fit In addition, the student learns photometric and vacuum techniques. Fluorescence quenching can be caused by a variety of physical and chemical processes involving collisions or other external perturbations which lead to the nonradiative demise of the fluorescing excited state. A bimolecular rate for collision-induced quenching can be determined by measuring fluorescence intensity as a function of collision partner concentration. The collision partner may be the ground state of the fluorescent species, i.e., self-quenchinn. -. or a different chemical species. i.e.. foreim-gas - . quenching. Experimental measurements and data analysis for a single quenching species can be accomplished in 4-5 ,hr, making this experiment suitable for 2-3 afternoons of lab. In our seuior-level lab, the experiment takes the format of an extended project allotted eight afternoons. This project involves writing and testing a weighted least squares program, detailed consideration of error compounding, selfand foreign-gas quenching measurements, and study of a theoretical model of the quenching collision. Since foreign-gas quenchers are innumerable, each student performs his own variation. In addition, a more ambitious project could investigate fluorescence polarization effects. The structure and spectroscopy of Iz is well documented in several sources (I), including the recent authoritative overview of Iz excited state theory by Mulliken (2). The He-Ne laser-excited fluorescence spectrum has been identified (3, 4). Quenching in Iz fluorescence has been thoroughly reviewed by Steinfeld (5), who has also recently measured (with colleagues) self-quenching in the He-Ne laser-excited fluorescence (6). Other Ip qnenching references provide additional insight (7). Absorbtion of the 632.8 nm radiation of the He-Ne laser excites 4 via the transitions described in Table 1. The absorbtion coefficient for the P(33) 6-3 transition is 40 times greater than for R(127) 11-5 ( 4 ) , indicating that the excited Iz population is mostly Iz(BBro,+, u' = 6, J' = 32). This state fluoresces in the 630-60 nm region (3). The fluorescence consists of a series of P. R rotational doublets forming a u" progression. Spectral resolution of the fluorescence reauires considerablv more intensitv than this experiment provides. 1 To whom reprint requests should be
858 /Journal of Chemical Education
addressed.
Predissociation in
12
The excited state may relax nonradiatively by spontaneous predissociation and, in the presence of collision partners, by collision-induced predissociation (2, 5). Predissociation occurs under the rather unusual condition that a repulsive electronic state has the same energy as the excited state, a given rovibrational level of a bound electronic state, in the region of highest Franck-Condon density (classical turning point of the vibration) (8). If certain symmetry requirements are satisfied, the true quantum state for the excited molecule is a combination of the bound and repulsive states, giving a certain probability for dissociation even though the excited state energy is less than the hound state dissociation energy. Spontaneous predissociation in I2 is caused by a combination or mixing of the bound B state with a repulsive 1s"state (5). The primary collision-induced predissociation for Iz involves mixing the B state with a repulsive 3ro,+ state. This mixing, forbidden by symmetry in the isolated molecule, is caused by a nonsymmetric van der Waals' interaction between I2 and the collision partner (5, 9). This mechanism predicts that the probability of quenching per collision is directly proportional to
where I, and a are the ionization potential and polarizability of the quenching collision partner, p is the reduced mass of the 12-collision partner system, and R is the mean distance of closest approach in the collision. Both spontaneous and collision-induced predissociation processes produce 3P3,z atoms. Stern-Volmer Quenching Model
Fluorescence qnenching is readily described in terms of the steady state kinetics of competing processes, the Stern-Volmer model (5, 10). Consider the following photochemical processes in Iz vapor irradiated with an excess of 632.8 nm photons
IP + h"
Absorbtian
I,*
Fluorescence
k h
Photodecomposition I$
+
k,
12*
(2)
+ hv'
(3)
21(2P,,,)
(4)
+
I,
k
Quenching I,* M -% 21(ZP8,,)+ M (5) We assume that the products of eqns. (4) and (5) remove excess energy from the reactions. Absorbtion and emission intensities (in photons cm+ sec-') are I. = k.Pz] and 1, = ktW1. Table 1. l2Absorption Transitions Leading to He-Ne Laser Exciled Fluorescence
X%+
,,
= 3.2, = 33
0 ' '
= 5 . 3 = 127
U28nm 632.8 nm
*
r
B Vlou+
Shorthand Notation
u'=6.P=32 u'Xl1.P = 128
P(331 6-3
R(127111-5
For self-quenching, M is Iz and the steady state production and disappearance of Iz* gives I,-'
=
K([IJ1
where K = (kr
+ Q)
(6)
+ hd)lklk,
,'q ,1 11: 1
---LASER DOPPLER LINE SHAPE
%
'\
-LASER
MODES
Q is known as the quenching constant and is the ratio of intercept to slope of I/-' plotted as a function of [la]-', as in Figure 3. If M is a foreign gas, then the quenching constant Q is determined by the ratio of slope to intercept of the function I,-' = K P(I + Q[Ml) (7) measured a t constant [Iz]. The Stern-Volmer description assumes that self-absorhtion, the ahsorbtion of hv' by Iz, and pressure broadening are negligible (11). Analysis of the fluorescence spectrum (3) indicates that the most intense fluorescence lines involve u" = 4-7 levels of the ground electronic state. Fluorescence ahsorbtion is negligible due to the small Boltzmann populations of these vibrational levels. Broadening of the Iz absorbtion lines would alter the spectral overlap with the exciting laser line, causing the absorbtion, and thus K and K' in e m s . (6) . . and (7). . . . to denend uDon the total gas pressure. his broadening is not dhserved in IZ absorhtion for inert gas pressures up to 100 Tom (12). Effective Quenching Diameter In the absence of quenching, the effective Iz* lifetime is found from
re/, is 0.31 p e c by direct decay measurement (13). The quenchingrate constant k, is thus
k = -Q 7111
From bard sphere collision theory (14)
where fi is the reduced mass of Iz* and collision partner and a, is the effective quenching diameter.= Fluorescence Excitation
-
Figure 1 illustrates the spectral coincidence of the 632.817 nm Ne emission line with the u' = 6, S = 32 v" = 3, J" = 33, and u' = 11, 3' = 128 v" = 5, J" = 127 ahsorbtion lines of Iz (4). General purpose He-Ne lasers oscillate simultaneously in more than one longitudinal cavity resonance mode (17). Output intensity is distributed among these modes according to the Ne Doppler line shape, which has a full width a t half intensity of ahout 900 MHz in a He-Ne laser. Longitudinal cavity modes are separated by approximately Au = c/2L, where c is the velocity of light and L is the distance between cavity reflectors ( I n . L and Av are typically 30 cm and 500 MHz in inexpensive low power lasers such as those used here. Absorhtion intensity I, is proportional to the spectral overlap of laser output and absorbtion lineshape as indicated schematically in Figure 1. Laser modes oscillating at frequencies lower than the center of the Ne emission are most effective in exciting fluorescence.
-
Figure 1. Schematic of i 2 absorbtion of He-Ne laser emission. Longitudinal laser modes for 30 cm laser cavity are arbitrarily centered under the Doppler lineshape. I? absorbtion of laser output is proportional to the Shaded areas. The i 2 absorbtion profile is from Ref. ( 4 ) .
In inexpensive He-Ne lasers, mode frequencies are not constant, hut continually change due to fluctuations in cavity length. Thus the overlap of laser output and absorbtion profile changes with time, causing I. and I/ to vary as much as 70% even though the overall laser intensity is stable to within 2%. T o circumvent the difficult task of stabilizing the laser output frequencies, the laser beam irradiates two fluorescence cells. One cell serves as a reference with fixed Iz pressure while the other contains Iz and M a t varying pressures. Fluorescence If from this second cell is measured relative to the reference fluorescence I,, thus compensating for changes in the laser spectral distribution. Attenuation of the laser intensity by reference cell absorbtion is less than 1%. Experimental We have used Lasers having rated outputs of 1.0 and 0.5 mW (University Labs Model 240, $295; Spectra-Physics Model 155, $100). Several other inexpensive lasers of camparahle power and mode spacing are on the market. A warmup period of 12-15 hr slaws the laser frequency fluctuation to a rate convenient for recording fluorescence intensities. The experimental setup is shown in Figure 2. The fluorescence cells are of amateur construction from 23 mm i.d. F'yrex tubing. Windows are %in. thick polished Pyrex discs (Esco Optics) fused'to the cell with no special precautions to prevent optical distortion. The reference cell has an exit window to allow the laser beam to pass into the sample cell, which terminates the beam in a tapered and curved end known as a Wood's horn (18) to minimize backscatter. The cell exteriors are painted black. Each window transmitting the laser beam is also painted except ZIt should be noted that the usage of o,z as the "quenching crass section" in the literature (5, 15, 16) is somewhat confusing because the actual cross section of the cylindrical hard sphere collision region swept hy an Iz+ molecule in 1 sec is scq2.(Reference (5)has omitted a s-' from the definition of ees.) Volume 50, Number 12, December 1973 / 859
Figure 2. Apparatus schematic. A, low-power He-Ne laser; B, reference 12 vapor cell; C. variable pressure l z vapor cell with Wood's horn radlation trap: D, photomultiplier; E, regulated high voltage power supply; F, recording electrometer.
for a 3-mm beam aoerture t o minimize scattered light. The laser and cells are clamped rigidly to the same mount. Fluorescence is measured with photomultiplier tubes mounted a t sidearm windows. The angular viewing aperture is approximately 0.1 rad. Scattered laser light a t the fluorescence detectors is typically 10% of I, a t 30 p of 12 pressure. The cells are sealed with high vacuum Teflon@ stopcocks and'have sidearms for controlling I2 vapor pressure using cold baths. We use RCA 6217 photomultiplier tubes in homemade housings consisting of socket, vinyl tape, and aluminum foil. The fluorescence falls within 3-10 or S-20 spectral response regions, necessitating relatively expensive (6217 tubes cast about $110) photomultipliers. Nominal cathode potentials of -7M) V are provided by ordinary regulated power supplies. There is no need to match the sample and reference photomultiplier gains. Photomultiplier anode currents are simultaneously measured on recording Heath EU-20-28 Log/Linear Current Modules. The fluorescence cells are filled in a grease- and Hg-free vacuum because of Iz reactivity. The vacuum system should provide a background pressure of less than 10 p . High vacuum Teflon@ stopcocks are used throughout. Pressure measuring devices using Hg should only be used with a protective Dry Ice o r liquid nitrogen trap. Reagent grade crystalline I. is vacuum sublimed to remove volatile impurities. 11 pressures are established by solid-vapor equilibrium temperatures (19). Constant temperature baths ranging from -10 to 20°C provide a useful range of Iz pressures. To perform self-quenching measurements, the reference cell sidearm is held a t 20°C while an appropriate constant temperature bath is applied to the sample eell sidearm. Pressure equilibrium in the sample cell can be detected by observing the recorder traces of the reference and sample cell photomultiplier currents. When equilibrium has been reached, the photomultiplier currents are noted simultaneously. The sidearm temperatures of both cells are then lowered to -78°C to reduce the 1. pressure t o
