- Email: [email protected]
Determination of excited state lifetimes and ionization potentials by dual beam visible lasers
Chemical Physics 42 (1979) 379-387 0 North-Holland Publishiig Company
DETERMINATION OF EXC ITED STATE LIFETIMES AND IONIZATION POTENTLALS BY DUAL BEAM VISIBLE LASERS
David H. PARKER and M.A. EL-SAYED Departtnent of Chemistry, University of Cakfornia, Los Angeles, California 900.24, USA
Received 20 Mach 1979
A two-laser multiphoton ionization (MPI) technique is used to measure the lifetime and ionization potential of the lowest excited Rydberg state of 1,4diazabicyclo[2,2,2] octane (DABCO). In this method one laser causes a two-photon transition to the excited state, and a second laser, at variable time delays from the frost, ionizes only the excited state molecules. Good agreement is found between the fluorescence lifetime of this two-photon state and the lifetime determined from the ionizing pulse delay. By scanning the ionizing laser wavelengthat a fixed time delay, the ionization threshold is obtained. The dual-beam MPI threshold is sharper than the onset of the photoelectron spectrum. This is due to the fact that transitions between Rydberg levels and the ionization continuum (which determine the shape of the threshold in this technique) are more Franck-Condon allowed than the transition between the ground state of the molecule and the ground state of the ion (observed in the photoelectron spectrum). This two-laser MPI method, which requires neither fluorescence, vacuumUV nor photoelectron equipment, should be applicable to a wide range of molecule;. A simple qualitative picture of DABCO MPI dynamics has also been constructed from an analysis of the dependence of the fluorescence intensity and the delayed ion current on the laser light intensity. Single-LaserMPI spectroswpy is shown to be limited by competition between ionization and dissociation or other quenching processes in the levels preceding the ionization continuum. The dual-beam MPI technique can be used in such a manner as to bypass dissociative states.
1. Introduction The laser has made possible the study of an entirely new class of phenomena involving the absorption of more than one photon. Initial observations and much of the development of present experimental and theoretical treatment of multiphoton processes came through the investigation of a common end result of such events - multiphoton ionization (MPI). The field originated, grew rapidly, and is no-w well established in the study of multiphoton ionization in atoms, particularly alkali metal atoms and their dimers. Many interesting aspects of atomic MPI have been reviewed, most recently by Iambropoulos [ I]. Multiphoton ionization was first-applied to stable molecules by Johnson et al. [2] and Petty et al. [3] in 1975. These authors and other workers [4] have shown MPI to be a sensitive probe of the electronic structure of highly excited molecules. In a typical experiment the absorption of from three to five iden-
tical photons from a pulsed tunable dye laser excites a gas-phase molecule into the ionization continuum. When the energy sum of two or three of these photons matches the energy of a two- or t!lree-photon allowed excited state the ionization cross section is greatly enhanced. By scanning the dye laser wavelength these highly excited states are traced out as resonances in the ionization spectrum. Another recent technique using high-intensity pulsed lasers is sequential two-step ionization. Here one laser brings a molecule to an excited state from which the second laser causes ionization, without ionizing ground state molecules. By varying the time delay between the excitation and ionization pulses the decay of the excited state population can be measured, as previously shown by Letokhov for formaldehyde [S]. By scanning the frequency of the ionizing laser, ionization potentials can be obtained as the sum of the threshold frequency of the second laser and the excited state energy supplied by the first laser. Measurements of
380
DJI. Parker. M.A. El-SayedlLiferimes and ionization potmials of DABCO
ionization potentials and lifetimes of uranium atoms have been reported using similar two-step [6] and. multi-step [7] ionization techniques. In this paper we use the combination of these two techniques, time-resolved two-step ionization and muhiphoton excitation, to study the spectroscopy and dynamics of the molecule 1,4-diazabicyclo[2,2,2] octane (DABCO). Two-photon excitation offers several improvements to t!le two-step ionization experiment. The previous two-step techniques involved one-photon excitation where an electronic transition was coincident with a fixed (UV) laser wavelength. Currently available visible-near UV tunable dye lasers can reach states from 3.5 eV to 7 eV with two-photon excita-
tion and states lying within 3.5 eV of the ionization potential can be ionized with the same type of laser. Most molecular Rydberg states [8], to which singlelaser MPI spectroscopy is particularly sensitive,should fall within this ionizing range. Since Rydberg geometries closely resemble those of the ion, ionization processes measured from an excited Rydberg level will be more Franck-Condon allowed than ionization processes from the ground state. This leads to a sharper and more clearly defined threshold. Valence levels with higher term values (which do not fit into a Rydberg series) can be ionized with the doubled frequency of the dye laser or with other UV laser sources. If short-wavelength sources are unavailable it should be possible in most cases to ionize excited states through sequential absorption of longer wavelength photons without exciting ground state molecules. The two-step MPI technique should thus be applicable to a wide range of molecules, and often with purely visible radiation. DABCO, which is a bicyclic caged amine, has been previously examined by MPI spectroscopy [9,10]. Besides possessing a relatively low ionization potential (7.23 eV) [ Ill, DABCO’s high (DJh) symmetry forces strict selection rulesfor transitions to the lowest excited states, most of which can be described as localized Rydberg excitations of the nitrogen lone pair orbitals [8,12]. Using the MPI polarization technique [4,13], the lowest singlet excited state (at 4.44 eV) has been assigned [lo] as A;, which is one-photon forbidden, two-photon allowed. This forbidden nature of the lowest excited state results in a long (~1 ~_ls) lifetime. The fluorescence quantum yield from this state is near unity [ 141. Lifetimes obtained by the dual-
(a) 1 (b)
-
Fig. 1. Experimentalapparatus: (a) for simultaneous measurement of tluorescenceand ionization caused by a single laser (laser 1). @) Additional Iaser(laser 2) with timiig delay for ionization of moleculesexcited by laser 1. Abbreviations: L-lens,F-fnter, M-mirror,GSglass slide. PD-photodiode, PMT-photomultiplier
tube, VRE-vibrating
reed electrometer.
beam experiment can thus be checked against the fluorescence lifetime. Furthermore, since the photoelectron spectrum of DABCO has been carefully analyzed [I 11, a direct comparison with the ionization threshold determined by dual-beam MPI is possible_ Since DABCO does emit from the same state which is ionized, a study of the fluorescence can be coupled with the dual-beam and singIe-beam MPI results to form a more complete picture of DABCO MPI dynamics. In particular we hope to find the rate limiting step in the two-photon resonant four-photon ionization with a single laser and further discuss the relative sensitivity of MPI spectroscopy to molecular Rydberg states.
2. Experimental The experimental apparatus used to compare fluorescence intensity with the ion current is first discussed along with the fluorescence lifetime measurement. Dual-beam experiments for determining excited state lifetimes and ionization potentials are then described. DABCO was obtained from the Aldrich Chemical Company and purified by vacuum sublimation. A small amount of solid in equilibrium with its room temperature vapor pressure was present in the otherwise evacuated cell.
D.H. Parker,MA. EMayed/Lifetimesand ionizationpotentialsof DAK0
Fluorescence and ionization were measured simultaneously with the apparatus shown in fig. la. Laser 1, a Molectron UV 1OOO/DL200 nitrogen pumped dye laser, was tuned to 558.74 nm (one-half the O-O band energy of the lowest excited state) and focused between the parallel plates of a glass ionization cell. The focusing lens (Ll) represents a range of focal lengths, from 5.5 to 55 cm. A 1 inch quartz window (not shown) in the side of the ceti ailows viewing of the 280-350 nm fluorescence [14] by a photomuitiplier tube @‘MT,RCA 8575 2” photocathode, at 2000 V (HV2)) which was isolated by a UV transparent-visible absorbing filter (F, Corning 7-54). The resulting fluorescence intensity and lifetime are then
determined by a boxcar averager. Ion current between the SO-400 V biased @VI) electrodes was measured
by a vibrating reed electrometer (VRE) and plotted along with the boxcar output on a dual pen chart recorder. Measurement of lifetimes longer than =I00 ns by dual-beam ionization requires two separate pump lasers delayed electronically with respect to each other. Without special triggering techniques [7] this can introduce significant timing jitter. Shorter lifetimes, spectroscopic and ionization potential determinations are best carried out with two optically delayed dye lasers excited by a single pump laser. For DABCO the timing jitter is less than 10% of the lifetime and results only in a reduction of the signal-to-noise ratio. In fig. lb the second laser (laser 2), a Quanta-Ray DCR Nd : YAG laser with second, third and fourth harmonic generation (532,355,266 nm), is included. The two laser pulses propagate in opposite directions along the same optic axis at variable temporal separations. Both lasers were scattered off a glass slide (GS) positioned near the cell onto a photodiode (PD) whose output was displayed on an oscilloscope. Pulse separations were monitored on the oscilloscope and adjusted with a simple pulse delay box, which triggered each laser. The total jitter in pulse separations with this arrangement was approximateIy SO ns, and the repetition rate was 10 Hz. .l.aser intensities and lens focal lengths were adjusted in the dual-beam experiments to maximize the ion current from the dual-beam process while minirnizing direct ground state ionization by either laser acting alone. It was found best to simply collimate the ionizing laser (laser 2) while focusingthe other laser
381
used to excite the two-photon state (laser 1). Lifetime measurements were carried out with laser 1 at 558.74 nm, 3.5 X 10e4 J/pulse, focused by Ll to a small spot (
(532 nm, 2.33 eV). The fourth harmonic (265 nm, 4.66 eV) is directly absorbed by the ground state and caused two-photon ionization at all workable intensities. Ionization potentials are determined from the dependence of the delayed ionization signal on the frequency of the ionizing laser. Since only laser 1 can be scanned, laser 2 was used to excite with the second harmonic (532 nm) into the higher vibrational levels of the lowest (two-photon) excited state. Laser 1 (now the ionizing laser) followed 400 ns later in order to insure relaxation (~0.5 Torr pressure) into the vibrationless level of the excited state. In this experiment, laser 2 is now focused (L2, SO cm) and the collimated laser 1 (~5 mm diameter) is aligned along the excitation laser axis. The excitation laser intensity was =10-3 J/pulse while the ionizing laser intensity was maintained at 1.2 X 10m4 J/pulse at wavelengths from 400 to 500 nm. Intensity dependences for each type of experiment were determined by first measuring the incident laser power with a calorimeter (Scientech 360) and then attenuating either laser beam before the focusing lens
and cell with calibrated neutral density filters.
3. Results 3.1. Lifetime determination The excited state decay obtained with the dualbeam MPI technique is compared with the fluorescence decay in fig. 2. Experimental conditions (sample pressure, excitation wavelength and intensity, and lens
D.H. Parker. M.A. El-Sa~edfL~ctimc~and
zTIME -0
'
I
(yet) I
ionization potentialsof
DABCO
4 5 6
- I‘
- IC
-I
I Fig. 2. CompaAon of (a) the fluorescence decay, and (b) the excited state ionization current as the ionizing laser is delayed from the excitation laser. Both slopes yield a lifetime of 820 + 20 ns.
focal length for laser 1) were the same for each method, both of which yield an S1 lifetime of 820 * 20 ns. Further agreement between the two methods is found in the dependence of the fluorescence and delayed beam ion current (both of which are proportional to the concentration of excited state molecules) on the excitation laser (laser 1) intensity. Both the fluorescence intensity and delayed ionization measurements were made ~500 ns after the excitation laser pulse. Laser 1 was attenuated by rieutral density filters to obtain the fluorescence intensity dependence shown in fig. 3a (laser 2 was not used). In fig. 3b the dependence of the delayed beam ion current on the excitation laser intensity is plotted. This is obtained by holding the ioniiing laser (laser 2, 355 nm) intensity constant while again attenuating laser 1. Over the given laser energy range (10 cm focusing lens) both signals follow an I1-‘*OV1dependence for the two-photon excitation by laser 1. The implications of this devikion from the expected I2 dependence on the overall excitation dynamics are discussed in section 3.3. As expected for a one-photon process, the delayed
IL1 I 10
’ ’ ’
’
’
LASER
I,,,,
’
,
I
I
1
.I INTENSITY
(orbit.
units)
Fig. 3. Comparisonof (a) the fluorescence intensity, and (b) the excited state ionization current (with the ionizing laser intensity held constant) as the excitation laser is attenuated. Both plots show an 11-7f0.1 dependence. In (c) the excitation laser intensity is held constant while the ionizing laser (355 nm, one-photon ionization) is attenuated. An ?-ok’.’ dependence is seen. ionization signal shows a linear dependence on the 355 nm ionizing laser (laser 2) intensity. In this measurement, plotted in fig. 3c, laser 1 is held constant while laser 2 is attenuated. Delayed beam ionization with laser 2 at 532 nm follows an 12*o’o*2 dependence on laser 2 (laser 1,558.7 run, constant I) over the limited range of intensities where ground state (4-photon) ionization does not overwhelm the twophoton ionization of excited state molecules, a problem which should be easily overcome by using ionization beam wavelengths longer than 560 run. 3.2. Ionization potentials A plot of the excited state delayed ionization current versus the ionizing laser wavelength is shown in fig. 4. Superimposed on this plot is the photoelectron
D.H. Parker,MA. El-Sayed/Lifetimesarrdionizationpottwtiakof DAK0
383
3.3. Excitatiort dynamics
Fig. 4. Ionizationpotential determined from the wavelength dependence of ionization from the lowest excited state (solid line) and from the photoeIectron spectrum [LT], which has been shifted by the excited state energy (4.44 eV) for comparison. of the lowest ionization potential of DABCO, shifted by the excited state energy (4.44 eV), as
spectrum
traced from the spectrum of Heilbronner and Muskat [15]. As seen in this figure, a sharp increase in excited
The preceding sections have emphasized the investigation and utilization of three-photon ionization in DABCO,where the third photon is provided by a second laser. This two-!aser teclmique is useful in both spectroscopic (e.g., ionization potentials) and dynamical (lifetime) studies of highly excited molecules. Direct ionization by a single laser (4-photon ionization in the case of DABCO), is experimentally simpler and thus more common. In this section the two-laser MPI technique will be coupled with a study of DABCO fluorescence processes in order to examine the act&excitation dynamics of the single-laser ionization event. Two basic and related questions to be considered are: (1) How fast is photoionization from the intermediate state as compared to rotation, dissociation, and other processes, and (2) which properties of the resonant intermediate states most greatly affect the ionization probability? The dynamics of any multistep excitation process, including resonance enhanced MPI, are best understood by monitoring the concentration of each successively excited species as the event proceeds. DABCO is an excellent molecule for such a study since fluores-
cence can be used to monitor the population of the first excited state (after the pulse) and ionization monitors
state (observed in the photoelectron experiment). In addition, laser studies have higher energy resolution
the final state. Only the third level in the four-photon ionization process cannot be directly probed. The major limitation to a quantitative measurement of the dynamic parameters, however, lies in the uncertainty of the laser intensity in the interaction volume. This is due mainly to the necessity of focusing the laser to obtain sufficient flux to observe these multiphoton events. Because of the poor coherence qualities of high intensity pulsed lasers and the use of imperfect focusing lenses, the beam radius at the focal point, wo, will be larger and the confocal parameter, b. (the axial length where the intensity is constant within 2u2), will be smaller than the values predicted [16] for a perfect gaussian beam. Even with an accurate measure of the cross section and volume of highest intensity region, it is still difficult to discriminate against signals arising from other less focused but physically much larger regions lying along the beam &is. For these reasons this analysis must remain qualitative in nature.
than the photoelectron experiment (1 cm-l compared to 100 cm-l).
dependence of the fluorescence. Since the emitting
state ionization begins at wavelengths shorter than =450 nm (2.76 eV), which correlates well with the adiabatic ionization potential of 7.23 eV. An even sharper threshold is expected if excitation by laser 2 bad a frequency corresponding to the O-O band directly, instead of relying on (incomplete) vibrational relaxation from the bigber vibrational levels excited in this study. In the future, when we obtain the Quanta-Ray dye laser system this will be accomplished. Even with the present experimental conditions, the energy threshold for ionization is still much sharper than the observed photoelectron threshold. This results from the fact that the transition from a Rydberg level to the ion ground state (as observed in our experiment) is more Franck-Condon allowed than that from the ground neutral state to the ground ionic
The first measurement to consider is the intensity
384
D.H. Parker,MA. El-Sayed/l$etitnes arldionizationpotetttialsof DABCO
state is populated by two-photon absorption, an Z2
dependence is expected in the absence of other processes. As seen in fig. 3, however, an /1-7*o*1 dependence of’the fluorescence and also the excited state ionization current (both measured 500 ns after the excitation pulse) was found when the laser was focused with a 10 cm lens. In this case the beam length along which the fluorescence (and ionization) was collected is much longer than the measured confocal length (1 mm or l/l0 the calculated maximum value). Using a longer lens v= 33 cm) with an actual confocal length oiroughly the order of the collection length and a relative flux of one-tenth the 10 cm lens (I a area -1 oc,-2 mfm2) an 11*5’o*1 dependence was found over idecade of intensity, which dropped to an 11*3*o-1dependence when an iris was used between the focal point and the PM tube to reduce the collection length. At approximately 0.03 mJ or 10% of the light initially incident on the 33 cm lens, the fluorescence begins to drop off much more rapidly with decreasing intensity, following an 12-o’o-2 dependence. The same general trends mentioned above were observed over a range of lens focal lengths from 5 to 45 cm. Under the same conditions both direct and delayed beam ionization can be measured. These additional measurements can be used to eliminate several mechanisms which should give similar fluorescence results. One possible explanation for the less than quadratic intensity dependence is total volume saturation, where every molecule in the interaction region is excited. in this case an overall I’*’ dependence must be followed, regardless of the number of photons used for excitation [17]. Several observations preclude this possibility: (1) At the highest intensity the total corrected amount of emission (or ionization) (
sections and thus will increase G to unity, is therefore negligible with the laser powers used in these measurements., Another possible mechanism could involve excited molecule interactions such as singlet-singlet annihilation, i.e. S, + S1 + S1 + S**, where S1 is the excited state, S, is the ground state and S** is a doubly excited state. This interaction must occur on a short time scale since the fluorescence (and the excited state ionization) follows a single exponential decay (fig. 2) within the experimental uncertainty (-20 ns). Fdr an excited molecule to collide with another excited molecule during the excitation laser pulse length, an unreasonably large cross section (-600 A2) is required. Thus, due to the low pressure, the small excited state concentration (
D.H. Parker, MA. El-SayedfLifetimes and ionization potentials of DABCO
385
dence of this quenching can be found. The rate equations for the populations of level 2 and level 3 are
These equations can be solved using a Laplace transform method and a square pulse approximation. It is found using the same derivation as Bradley [18] (with only the addition of k, in fig. 5), that the total excited state population (in levels 2 and 3) immediately after the pulse ends is given by: N*(T) = N2 + fv3 = Noa@
Fig. 5. Kinetic scheme for the four-photon ionization of DABCO by a single laser, where the fust two photons are resonant with the lowest excited state (Ievel2).
can fluorescence at a rate k~ = 117~ (cl.2 X lo6 s-l) or absorb another photon at of&_ The third photon lies in a region of dense absorption and is thus resonant with another highly excited state (level 3). From level 3 the molecule can either dissociate (i.e. be lost from the system) at a rate kL, relax back to level 2 at the rate kR, undergo stimulated emission at j@f back to level 2, or absorb a fourth photon into the ionization continuum. Since the population of level 2, N2, is always a small fraction of the ground state population, No (shown later), stimulated emission from level 2 back to the ground state (moleculnr rate N2$$Z2) is not included. The experimentauy obse-ved loss of emitting molecules is due to the term; k, and a$$)1 in the rate scheme. Fluorescence quenching becomes likely as the product of the loss rate and the laser pulse length approaches unity, that is when u$yIT or kLT2 1, where T is the pulse length. In the absence of these processes the excited population will simply distribute itself between levels 2 and 3 until the laser pulse ends, at which the level 3 population relaxes rapidly (rate kR) into level 2, which in turn emits slowly at the rate kF into the ground state. Loss must occur through level 3 since the fluorescence quantum yield of level 2 is unity using conventional (low intensity) one-photon excitation methods [14]. By solving the rate equations for the processes shown in fig. 5, the intensity depen-
where T is the laser pulse length and WZ and IV3 are complicated functions of the laser intensity and the other rate parameters. The second quantity in the curly brackets represents the deviation from 1’ of the fluorescence [which is proportional to N*(T)]. At low light levels IV2 goes to unity and W, approaches zero so no quenching results. At high light levels IV2 approaches zero and IV3 goes to (u~~u&~ + uj4kLT)-] under the conditions (kL + 03~1) 2 kR and (~23 +&VT9 1. As will be discussed later, these conditions should be met in the higher intensity range of this experiment. At these intensitiesN*(T) becomes N*(T) = N~u#I~T[(u~~ X (q39.d2
+ fi32 + U34y + kR + kL]
+ q4k&,
which predicts a less than quadratic fluorescence intensity dependence. At even higher intensities the fluorescence varies linearly with the laser flux. These predicted trends agree with the observed fluorescence behavior. Several other observations can be coupled with the above rate scheme to form a more complete picture of DABCO photoexcitation dynamics: (1) The fraction of the excited state population which is ionized during the laser pulse can be estimated by the ratio of the direct (single-laser) ion current over the delayed beam (one-photon ionization) current. This ratio will be at least one order of magnitude too large since for all focusing lenses the delayed current is not saturated (fig. 3c). Ionization accounts for a significant fraction [zlO% as estimated by (& Ion./ideI. Ion.) X (j-11 of the
386
D.H. Parker. MA. N-SayedjLifetims
excited molecules with only the shortest (a5 cm) focusing lenses. With lenses of longer focal length (same number of photons) the ionized fraction becomes increasingly small, from 0.1% forf= 15 cm to 0.01% for f = 33 cm. Under the same conditions, however, fluorescence quenching is still efficient, indicating that for all but the highest fluxes ionization is not the major loss factor. (2) When DAK0 is excited into the region of the third photon resonance of this MPI experiment by direct two-photon absorption, no fluorescence and only a small ion current is observed. The loss rate from level 3 is thus much greater than the relaxation rate (kL > kP), a fact not surprising considering the high energy (~6.8 eV) and diffuseness of this spectral region. As mentioned previously, the inverse of the loss rate need only be of the order of the pulse length for quenching from level 3 to occur. k, is certainly much larger than this minimum rate (~10’ s-‘) but not so large that ionization (rate u 5YI) does not compete at higher intensities. The above observations suggest that at intensities where quenching does not take place, as indicated by an 1’ fluorescence dependence, the population of level 3 is small. At the quenching threshold the probability of the third photon absorption during the laser pulse approaches unity, i.e. cr$yZT= I. The intensity at this point is roughly approximated as =lO*’ photons/cm’ s which coupled with the laser pulse length of NO-’ s gives a value for o&I as ~1O-l~ cm7. Although uncertain within at least an order of magnitude, this is a reasonabIe one-photon singletsinglet absorption cross section. If a similar value is assumed for o&l, then k, can be estimated at the point where ionization effectively competes with other quenching rates. This is the highest intensity region (I= 1O28 photons/cm* s) which upon substitution into k, w c@Tgives k, w 1011 s-l. If at this intensity 10% of the excited population is ionized, the total excited population, Ngti, is roughly ten times the measured ion current. Assuming a collection efficiency of unity, which is reasonable for the bias voltage and ion collection geometry used, this gives NY = 5 X lo8 moleciiles excited (per laser pulse). S~~,tot~=N&#~l*Z’~tiO being the initial number of molecules in the focal region, which for a 5 cm focusing lens is ~5 X lOI molecules), ~‘021can be estimated as =10-50 cm4 s. It must be emphasized that these values for #, kL and @, while reasonable,
and ionization potentials
of DABCO
are rough estimates, and can only be taken to lend support to the proposed dynamics. The overall picture of DABCO MPI dynamics can now be summarized. Fluorescence quenching which takes place at all but the Iowest intensities arises from further absorption from level 2 (the S, state) to a dissociative level (level 3). At high intensities, ionization from level 3 begins to overtake the other losses and becomes the major process up to total volume saturation. The ion current is thus Iimited by competition, in the third photon resonant level, from dissociation or other relaxation processes which remove the molecule from ionization or fluorescence. Such competition in the level(s) preceding the ion continuum, even in the two-photon state in a threephoton ionization, is likely to limit in any molecule the type of two-photon resonances detectable by single-beam MPI spectroscopy to long-lived states whose subsequent levels (if any) are also reasonably stable. This probably results in the particular sensitivity of the technique to molecular “Rydberg-type” states [4], whose generally sharp structure indicates little mixing with dissociative valence levels. By bypassing the dissociative intermediate levels (e.g. level 3 in fig. 5) with a W laser with one-photon excited state photoionization, the.spectroscopicaIIy interesting, low-lying (and longer-lived) “valence” states should thus become accessible. One further point can be made concerning DABCO MPI dynamics. As mentioned previously, the polarization ratio, a, which is determined by the symmetry of the two-photon transition can be measured directly through the ratio of/fluorescence arising from a circularly polarized laser over a linearly polarized laser. A one-photon transition does not have a polarization dependence for unoriented (gas-phase) molecules, i.e. !$I = @/& = 1. If the third and/or fourth photon absorption in the single-laser DABCO experiment takes place so fast, however, that the molecules excited by the initial two-photon transition still retain their preferred orientation with respect to the laser field, a non-unity S$) will be observed. The same overall !G? was found within experimental error for the ratio of the DABCO ion current as for the fluorescence, i.e. n MPI = &ror = I/40 (at a pressure of 0.5 Torr). Thus even though the ionization step follows an ~1’ dependence (two-photon ionization from the S1 state) the overall ionization process is biphotonic in charac-
D.H. Parker, MA. El-Sayed/Lifetimes and ionization potentials of DABCO
ter, and slower than the rotational disorientation
rate, for the intensity range of the experiment. It must be expected that at higher intensities or lower pressures than those used in this experiment ionization should overtake rotational relaxation. This observation will be complicated, however, by volume saturation.
4. Summary
The dual-beam MPI technique has been shown to be useful in measuring the S, lifetime and ionization potential of DABCO in a very simple way. This method which requires neither fluorescence nor vacuum W equipment should be applicable to a wide range of molecules. A qualitative picture of DABCO MPI dynamics has also been constructed by comparison of fluorescence and ionization. Single-beam MPI spectroscopy is seen to be limited by competition between ionization and dissociation or other quenching processes in the levels preceding the ion continuum. Such limitations should be overcome by the dual-beam MPI technique.
&knowledgement The authors would like to thank the National Science Foundation for their financial support.
References [l] P. Lambropoulos, Advan. At. Mol. Phys. 12 (1976) 87.
387
[2] P.M. Johnson, M.R. Berman and D. Zakheim, J. Chem. Phys. 62 (1975) 2500. [3] G. Petty, C. Tai and F.W. DaIby, Phys. Rev. Letters 34 (1975) 1207. 141 D.H. Parker, J.O. Berg and M.A. El-Sayed, in: Advances in laser chemistry, ed. A.H. Zewail, Springer Series in Chemical Physics (Springer, Berlin, 1978) p. 319. [S] S.V. Andreyev, V.S. Antonov, LN. Knyazev and V.S. Letokhov, Chem. Phys. Letters 45 (1977) 166. [6] C.S. James, I. Itzkan, CT. Pike, R.H. Levy and L. Levin, IEEE J. Quantum Electron. QE12 (1976) 111. [7] R.W. So&s, C.A. May, L.R. Carlson, E.F. Worden, S.A. Johnson, J.A. Paisner and L.J. Raidzieemski, Phys. Rev. A 14 (1976) 1129. [8] M. Robin, Higher excited states of polyatomic molecules, Vols. 1,2 (Academic Press, New York, 1974, 1975). [9] D.H. Parker and P. Avouris, Chem. Phys. Letters 53 (1978) 515. [lo] D-H. Parker and P. Avouris, in preparation. [ 1 l] P. Bischof, J.A. HashmalI, E. HeiIbronner and V. Homung, Tetrahedron Letters 46 (1969) 4025. [I2 ] A.M. Halpern, J.L. Roebber and K. Weiss, J. Chem. Phys. 49 (1968) 1348. [13] J.O. Berg, D.H. Parker and M.A. El-Sayed, J. Chem. Phys. 68 (1978) 5661. [14] A.M. H&em, Chem. Phys. Letters 6 (1970) 296. [15] E. Heilbronner and K.A. Muskat, J. Am. Chem. Sot. 92 (1970) 3818. 1161 A.E. Siegman, An introduction to lasers and masers (McGraw-Hill, New York, 1971). [17] S. Speiser and J. Jortner, Chem. Phys. Letters 44 (1976) 399. [18] D.J. Bradley, M.H.R. Hutchinson, H. Koetser, T. Morrow, G.H.C. New andM.S. Petty,Proc. Roy. Sot. London A 328 (1972) 97; D-T.Bradley, M.H.R. Hutchinson and H. Koetser, Proc. Roy. Sot. London A 329 (1972) 105.
DETERMINATION OF EXC ITED STATE LIFETIMES AND IONIZATION POTENTLALS BY DUAL BEAM VISIBLE LASERS
David H. PARKER and M.A. EL-SAYED Departtnent of Chemistry, University of Cakfornia, Los Angeles, California 900.24, USA
Received 20 Mach 1979
A two-laser multiphoton ionization (MPI) technique is used to measure the lifetime and ionization potential of the lowest excited Rydberg state of 1,4diazabicyclo[2,2,2] octane (DABCO). In this method one laser causes a two-photon transition to the excited state, and a second laser, at variable time delays from the frost, ionizes only the excited state molecules. Good agreement is found between the fluorescence lifetime of this two-photon state and the lifetime determined from the ionizing pulse delay. By scanning the ionizing laser wavelengthat a fixed time delay, the ionization threshold is obtained. The dual-beam MPI threshold is sharper than the onset of the photoelectron spectrum. This is due to the fact that transitions between Rydberg levels and the ionization continuum (which determine the shape of the threshold in this technique) are more Franck-Condon allowed than the transition between the ground state of the molecule and the ground state of the ion (observed in the photoelectron spectrum). This two-laser MPI method, which requires neither fluorescence, vacuumUV nor photoelectron equipment, should be applicable to a wide range of molecule;. A simple qualitative picture of DABCO MPI dynamics has also been constructed from an analysis of the dependence of the fluorescence intensity and the delayed ion current on the laser light intensity. Single-LaserMPI spectroswpy is shown to be limited by competition between ionization and dissociation or other quenching processes in the levels preceding the ionization continuum. The dual-beam MPI technique can be used in such a manner as to bypass dissociative states.
1. Introduction The laser has made possible the study of an entirely new class of phenomena involving the absorption of more than one photon. Initial observations and much of the development of present experimental and theoretical treatment of multiphoton processes came through the investigation of a common end result of such events - multiphoton ionization (MPI). The field originated, grew rapidly, and is no-w well established in the study of multiphoton ionization in atoms, particularly alkali metal atoms and their dimers. Many interesting aspects of atomic MPI have been reviewed, most recently by Iambropoulos [ I]. Multiphoton ionization was first-applied to stable molecules by Johnson et al. [2] and Petty et al. [3] in 1975. These authors and other workers [4] have shown MPI to be a sensitive probe of the electronic structure of highly excited molecules. In a typical experiment the absorption of from three to five iden-
tical photons from a pulsed tunable dye laser excites a gas-phase molecule into the ionization continuum. When the energy sum of two or three of these photons matches the energy of a two- or t!lree-photon allowed excited state the ionization cross section is greatly enhanced. By scanning the dye laser wavelength these highly excited states are traced out as resonances in the ionization spectrum. Another recent technique using high-intensity pulsed lasers is sequential two-step ionization. Here one laser brings a molecule to an excited state from which the second laser causes ionization, without ionizing ground state molecules. By varying the time delay between the excitation and ionization pulses the decay of the excited state population can be measured, as previously shown by Letokhov for formaldehyde [S]. By scanning the frequency of the ionizing laser, ionization potentials can be obtained as the sum of the threshold frequency of the second laser and the excited state energy supplied by the first laser. Measurements of
380
DJI. Parker. M.A. El-SayedlLiferimes and ionization potmials of DABCO
ionization potentials and lifetimes of uranium atoms have been reported using similar two-step [6] and. multi-step [7] ionization techniques. In this paper we use the combination of these two techniques, time-resolved two-step ionization and muhiphoton excitation, to study the spectroscopy and dynamics of the molecule 1,4-diazabicyclo[2,2,2] octane (DABCO). Two-photon excitation offers several improvements to t!le two-step ionization experiment. The previous two-step techniques involved one-photon excitation where an electronic transition was coincident with a fixed (UV) laser wavelength. Currently available visible-near UV tunable dye lasers can reach states from 3.5 eV to 7 eV with two-photon excita-
tion and states lying within 3.5 eV of the ionization potential can be ionized with the same type of laser. Most molecular Rydberg states [8], to which singlelaser MPI spectroscopy is particularly sensitive,should fall within this ionizing range. Since Rydberg geometries closely resemble those of the ion, ionization processes measured from an excited Rydberg level will be more Franck-Condon allowed than ionization processes from the ground state. This leads to a sharper and more clearly defined threshold. Valence levels with higher term values (which do not fit into a Rydberg series) can be ionized with the doubled frequency of the dye laser or with other UV laser sources. If short-wavelength sources are unavailable it should be possible in most cases to ionize excited states through sequential absorption of longer wavelength photons without exciting ground state molecules. The two-step MPI technique should thus be applicable to a wide range of molecules, and often with purely visible radiation. DABCO, which is a bicyclic caged amine, has been previously examined by MPI spectroscopy [9,10]. Besides possessing a relatively low ionization potential (7.23 eV) [ Ill, DABCO’s high (DJh) symmetry forces strict selection rulesfor transitions to the lowest excited states, most of which can be described as localized Rydberg excitations of the nitrogen lone pair orbitals [8,12]. Using the MPI polarization technique [4,13], the lowest singlet excited state (at 4.44 eV) has been assigned [lo] as A;, which is one-photon forbidden, two-photon allowed. This forbidden nature of the lowest excited state results in a long (~1 ~_ls) lifetime. The fluorescence quantum yield from this state is near unity [ 141. Lifetimes obtained by the dual-
(a) 1 (b)
-
Fig. 1. Experimentalapparatus: (a) for simultaneous measurement of tluorescenceand ionization caused by a single laser (laser 1). @) Additional Iaser(laser 2) with timiig delay for ionization of moleculesexcited by laser 1. Abbreviations: L-lens,F-fnter, M-mirror,GSglass slide. PD-photodiode, PMT-photomultiplier
tube, VRE-vibrating
reed electrometer.
beam experiment can thus be checked against the fluorescence lifetime. Furthermore, since the photoelectron spectrum of DABCO has been carefully analyzed [I 11, a direct comparison with the ionization threshold determined by dual-beam MPI is possible_ Since DABCO does emit from the same state which is ionized, a study of the fluorescence can be coupled with the dual-beam and singIe-beam MPI results to form a more complete picture of DABCO MPI dynamics. In particular we hope to find the rate limiting step in the two-photon resonant four-photon ionization with a single laser and further discuss the relative sensitivity of MPI spectroscopy to molecular Rydberg states.
2. Experimental The experimental apparatus used to compare fluorescence intensity with the ion current is first discussed along with the fluorescence lifetime measurement. Dual-beam experiments for determining excited state lifetimes and ionization potentials are then described. DABCO was obtained from the Aldrich Chemical Company and purified by vacuum sublimation. A small amount of solid in equilibrium with its room temperature vapor pressure was present in the otherwise evacuated cell.
D.H. Parker,MA. EMayed/Lifetimesand ionizationpotentialsof DAK0
Fluorescence and ionization were measured simultaneously with the apparatus shown in fig. la. Laser 1, a Molectron UV 1OOO/DL200 nitrogen pumped dye laser, was tuned to 558.74 nm (one-half the O-O band energy of the lowest excited state) and focused between the parallel plates of a glass ionization cell. The focusing lens (Ll) represents a range of focal lengths, from 5.5 to 55 cm. A 1 inch quartz window (not shown) in the side of the ceti ailows viewing of the 280-350 nm fluorescence [14] by a photomuitiplier tube @‘MT,RCA 8575 2” photocathode, at 2000 V (HV2)) which was isolated by a UV transparent-visible absorbing filter (F, Corning 7-54). The resulting fluorescence intensity and lifetime are then
determined by a boxcar averager. Ion current between the SO-400 V biased @VI) electrodes was measured
by a vibrating reed electrometer (VRE) and plotted along with the boxcar output on a dual pen chart recorder. Measurement of lifetimes longer than =I00 ns by dual-beam ionization requires two separate pump lasers delayed electronically with respect to each other. Without special triggering techniques [7] this can introduce significant timing jitter. Shorter lifetimes, spectroscopic and ionization potential determinations are best carried out with two optically delayed dye lasers excited by a single pump laser. For DABCO the timing jitter is less than 10% of the lifetime and results only in a reduction of the signal-to-noise ratio. In fig. lb the second laser (laser 2), a Quanta-Ray DCR Nd : YAG laser with second, third and fourth harmonic generation (532,355,266 nm), is included. The two laser pulses propagate in opposite directions along the same optic axis at variable temporal separations. Both lasers were scattered off a glass slide (GS) positioned near the cell onto a photodiode (PD) whose output was displayed on an oscilloscope. Pulse separations were monitored on the oscilloscope and adjusted with a simple pulse delay box, which triggered each laser. The total jitter in pulse separations with this arrangement was approximateIy SO ns, and the repetition rate was 10 Hz. .l.aser intensities and lens focal lengths were adjusted in the dual-beam experiments to maximize the ion current from the dual-beam process while minirnizing direct ground state ionization by either laser acting alone. It was found best to simply collimate the ionizing laser (laser 2) while focusingthe other laser
381
used to excite the two-photon state (laser 1). Lifetime measurements were carried out with laser 1 at 558.74 nm, 3.5 X 10e4 J/pulse, focused by Ll to a small spot (
(532 nm, 2.33 eV). The fourth harmonic (265 nm, 4.66 eV) is directly absorbed by the ground state and caused two-photon ionization at all workable intensities. Ionization potentials are determined from the dependence of the delayed ionization signal on the frequency of the ionizing laser. Since only laser 1 can be scanned, laser 2 was used to excite with the second harmonic (532 nm) into the higher vibrational levels of the lowest (two-photon) excited state. Laser 1 (now the ionizing laser) followed 400 ns later in order to insure relaxation (~0.5 Torr pressure) into the vibrationless level of the excited state. In this experiment, laser 2 is now focused (L2, SO cm) and the collimated laser 1 (~5 mm diameter) is aligned along the excitation laser axis. The excitation laser intensity was =10-3 J/pulse while the ionizing laser intensity was maintained at 1.2 X 10m4 J/pulse at wavelengths from 400 to 500 nm. Intensity dependences for each type of experiment were determined by first measuring the incident laser power with a calorimeter (Scientech 360) and then attenuating either laser beam before the focusing lens
and cell with calibrated neutral density filters.
3. Results 3.1. Lifetime determination The excited state decay obtained with the dualbeam MPI technique is compared with the fluorescence decay in fig. 2. Experimental conditions (sample pressure, excitation wavelength and intensity, and lens
D.H. Parker. M.A. El-Sa~edfL~ctimc~and
zTIME -0
'
I
(yet) I
ionization potentialsof
DABCO
4 5 6
- I‘
- IC
-I
I Fig. 2. CompaAon of (a) the fluorescence decay, and (b) the excited state ionization current as the ionizing laser is delayed from the excitation laser. Both slopes yield a lifetime of 820 + 20 ns.
focal length for laser 1) were the same for each method, both of which yield an S1 lifetime of 820 * 20 ns. Further agreement between the two methods is found in the dependence of the fluorescence and delayed beam ion current (both of which are proportional to the concentration of excited state molecules) on the excitation laser (laser 1) intensity. Both the fluorescence intensity and delayed ionization measurements were made ~500 ns after the excitation laser pulse. Laser 1 was attenuated by rieutral density filters to obtain the fluorescence intensity dependence shown in fig. 3a (laser 2 was not used). In fig. 3b the dependence of the delayed beam ion current on the excitation laser intensity is plotted. This is obtained by holding the ioniiing laser (laser 2, 355 nm) intensity constant while again attenuating laser 1. Over the given laser energy range (10 cm focusing lens) both signals follow an I1-‘*OV1dependence for the two-photon excitation by laser 1. The implications of this devikion from the expected I2 dependence on the overall excitation dynamics are discussed in section 3.3. As expected for a one-photon process, the delayed
IL1 I 10
’ ’ ’
’
’
LASER
I,,,,
’
,
I
I
1
.I INTENSITY
(orbit.
units)
Fig. 3. Comparisonof (a) the fluorescence intensity, and (b) the excited state ionization current (with the ionizing laser intensity held constant) as the excitation laser is attenuated. Both plots show an 11-7f0.1 dependence. In (c) the excitation laser intensity is held constant while the ionizing laser (355 nm, one-photon ionization) is attenuated. An ?-ok’.’ dependence is seen. ionization signal shows a linear dependence on the 355 nm ionizing laser (laser 2) intensity. In this measurement, plotted in fig. 3c, laser 1 is held constant while laser 2 is attenuated. Delayed beam ionization with laser 2 at 532 nm follows an 12*o’o*2 dependence on laser 2 (laser 1,558.7 run, constant I) over the limited range of intensities where ground state (4-photon) ionization does not overwhelm the twophoton ionization of excited state molecules, a problem which should be easily overcome by using ionization beam wavelengths longer than 560 run. 3.2. Ionization potentials A plot of the excited state delayed ionization current versus the ionizing laser wavelength is shown in fig. 4. Superimposed on this plot is the photoelectron
D.H. Parker,MA. El-Sayed/Lifetimesarrdionizationpottwtiakof DAK0
383
3.3. Excitatiort dynamics
Fig. 4. Ionizationpotential determined from the wavelength dependence of ionization from the lowest excited state (solid line) and from the photoeIectron spectrum [LT], which has been shifted by the excited state energy (4.44 eV) for comparison. of the lowest ionization potential of DABCO, shifted by the excited state energy (4.44 eV), as
spectrum
traced from the spectrum of Heilbronner and Muskat [15]. As seen in this figure, a sharp increase in excited
The preceding sections have emphasized the investigation and utilization of three-photon ionization in DABCO,where the third photon is provided by a second laser. This two-!aser teclmique is useful in both spectroscopic (e.g., ionization potentials) and dynamical (lifetime) studies of highly excited molecules. Direct ionization by a single laser (4-photon ionization in the case of DABCO), is experimentally simpler and thus more common. In this section the two-laser MPI technique will be coupled with a study of DABCO fluorescence processes in order to examine the act&excitation dynamics of the single-laser ionization event. Two basic and related questions to be considered are: (1) How fast is photoionization from the intermediate state as compared to rotation, dissociation, and other processes, and (2) which properties of the resonant intermediate states most greatly affect the ionization probability? The dynamics of any multistep excitation process, including resonance enhanced MPI, are best understood by monitoring the concentration of each successively excited species as the event proceeds. DABCO is an excellent molecule for such a study since fluores-
cence can be used to monitor the population of the first excited state (after the pulse) and ionization monitors
state (observed in the photoelectron experiment). In addition, laser studies have higher energy resolution
the final state. Only the third level in the four-photon ionization process cannot be directly probed. The major limitation to a quantitative measurement of the dynamic parameters, however, lies in the uncertainty of the laser intensity in the interaction volume. This is due mainly to the necessity of focusing the laser to obtain sufficient flux to observe these multiphoton events. Because of the poor coherence qualities of high intensity pulsed lasers and the use of imperfect focusing lenses, the beam radius at the focal point, wo, will be larger and the confocal parameter, b. (the axial length where the intensity is constant within 2u2), will be smaller than the values predicted [16] for a perfect gaussian beam. Even with an accurate measure of the cross section and volume of highest intensity region, it is still difficult to discriminate against signals arising from other less focused but physically much larger regions lying along the beam &is. For these reasons this analysis must remain qualitative in nature.
than the photoelectron experiment (1 cm-l compared to 100 cm-l).
dependence of the fluorescence. Since the emitting
state ionization begins at wavelengths shorter than =450 nm (2.76 eV), which correlates well with the adiabatic ionization potential of 7.23 eV. An even sharper threshold is expected if excitation by laser 2 bad a frequency corresponding to the O-O band directly, instead of relying on (incomplete) vibrational relaxation from the bigber vibrational levels excited in this study. In the future, when we obtain the Quanta-Ray dye laser system this will be accomplished. Even with the present experimental conditions, the energy threshold for ionization is still much sharper than the observed photoelectron threshold. This results from the fact that the transition from a Rydberg level to the ion ground state (as observed in our experiment) is more Franck-Condon allowed than that from the ground neutral state to the ground ionic
The first measurement to consider is the intensity
384
D.H. Parker,MA. El-Sayed/l$etitnes arldionizationpotetttialsof DABCO
state is populated by two-photon absorption, an Z2
dependence is expected in the absence of other processes. As seen in fig. 3, however, an /1-7*o*1 dependence of’the fluorescence and also the excited state ionization current (both measured 500 ns after the excitation pulse) was found when the laser was focused with a 10 cm lens. In this case the beam length along which the fluorescence (and ionization) was collected is much longer than the measured confocal length (1 mm or l/l0 the calculated maximum value). Using a longer lens v= 33 cm) with an actual confocal length oiroughly the order of the collection length and a relative flux of one-tenth the 10 cm lens (I a area -1 oc,-2 mfm2) an 11*5’o*1 dependence was found over idecade of intensity, which dropped to an 11*3*o-1dependence when an iris was used between the focal point and the PM tube to reduce the collection length. At approximately 0.03 mJ or 10% of the light initially incident on the 33 cm lens, the fluorescence begins to drop off much more rapidly with decreasing intensity, following an 12-o’o-2 dependence. The same general trends mentioned above were observed over a range of lens focal lengths from 5 to 45 cm. Under the same conditions both direct and delayed beam ionization can be measured. These additional measurements can be used to eliminate several mechanisms which should give similar fluorescence results. One possible explanation for the less than quadratic intensity dependence is total volume saturation, where every molecule in the interaction region is excited. in this case an overall I’*’ dependence must be followed, regardless of the number of photons used for excitation [17]. Several observations preclude this possibility: (1) At the highest intensity the total corrected amount of emission (or ionization) (
sections and thus will increase G to unity, is therefore negligible with the laser powers used in these measurements., Another possible mechanism could involve excited molecule interactions such as singlet-singlet annihilation, i.e. S, + S1 + S1 + S**, where S1 is the excited state, S, is the ground state and S** is a doubly excited state. This interaction must occur on a short time scale since the fluorescence (and the excited state ionization) follows a single exponential decay (fig. 2) within the experimental uncertainty (-20 ns). Fdr an excited molecule to collide with another excited molecule during the excitation laser pulse length, an unreasonably large cross section (-600 A2) is required. Thus, due to the low pressure, the small excited state concentration (
D.H. Parker, MA. El-SayedfLifetimes and ionization potentials of DABCO
385
dence of this quenching can be found. The rate equations for the populations of level 2 and level 3 are
These equations can be solved using a Laplace transform method and a square pulse approximation. It is found using the same derivation as Bradley [18] (with only the addition of k, in fig. 5), that the total excited state population (in levels 2 and 3) immediately after the pulse ends is given by: N*(T) = N2 + fv3 = Noa@
Fig. 5. Kinetic scheme for the four-photon ionization of DABCO by a single laser, where the fust two photons are resonant with the lowest excited state (Ievel2).
can fluorescence at a rate k~ = 117~ (cl.2 X lo6 s-l) or absorb another photon at of&_ The third photon lies in a region of dense absorption and is thus resonant with another highly excited state (level 3). From level 3 the molecule can either dissociate (i.e. be lost from the system) at a rate kL, relax back to level 2 at the rate kR, undergo stimulated emission at j@f back to level 2, or absorb a fourth photon into the ionization continuum. Since the population of level 2, N2, is always a small fraction of the ground state population, No (shown later), stimulated emission from level 2 back to the ground state (moleculnr rate N2$$Z2) is not included. The experimentauy obse-ved loss of emitting molecules is due to the term; k, and a$$)1 in the rate scheme. Fluorescence quenching becomes likely as the product of the loss rate and the laser pulse length approaches unity, that is when u$yIT or kLT2 1, where T is the pulse length. In the absence of these processes the excited population will simply distribute itself between levels 2 and 3 until the laser pulse ends, at which the level 3 population relaxes rapidly (rate kR) into level 2, which in turn emits slowly at the rate kF into the ground state. Loss must occur through level 3 since the fluorescence quantum yield of level 2 is unity using conventional (low intensity) one-photon excitation methods [14]. By solving the rate equations for the processes shown in fig. 5, the intensity depen-
where T is the laser pulse length and WZ and IV3 are complicated functions of the laser intensity and the other rate parameters. The second quantity in the curly brackets represents the deviation from 1’ of the fluorescence [which is proportional to N*(T)]. At low light levels IV2 goes to unity and W, approaches zero so no quenching results. At high light levels IV2 approaches zero and IV3 goes to (u~~u&~ + uj4kLT)-] under the conditions (kL + 03~1) 2 kR and (~23 +&VT9 1. As will be discussed later, these conditions should be met in the higher intensity range of this experiment. At these intensitiesN*(T) becomes N*(T) = N~u#I~T[(u~~ X (q39.d2
+ fi32 + U34y + kR + kL]
+ q4k&,
which predicts a less than quadratic fluorescence intensity dependence. At even higher intensities the fluorescence varies linearly with the laser flux. These predicted trends agree with the observed fluorescence behavior. Several other observations can be coupled with the above rate scheme to form a more complete picture of DABCO photoexcitation dynamics: (1) The fraction of the excited state population which is ionized during the laser pulse can be estimated by the ratio of the direct (single-laser) ion current over the delayed beam (one-photon ionization) current. This ratio will be at least one order of magnitude too large since for all focusing lenses the delayed current is not saturated (fig. 3c). Ionization accounts for a significant fraction [zlO% as estimated by (& Ion./ideI. Ion.) X (j-11 of the
386
D.H. Parker. MA. N-SayedjLifetims
excited molecules with only the shortest (a5 cm) focusing lenses. With lenses of longer focal length (same number of photons) the ionized fraction becomes increasingly small, from 0.1% forf= 15 cm to 0.01% for f = 33 cm. Under the same conditions, however, fluorescence quenching is still efficient, indicating that for all but the highest fluxes ionization is not the major loss factor. (2) When DAK0 is excited into the region of the third photon resonance of this MPI experiment by direct two-photon absorption, no fluorescence and only a small ion current is observed. The loss rate from level 3 is thus much greater than the relaxation rate (kL > kP), a fact not surprising considering the high energy (~6.8 eV) and diffuseness of this spectral region. As mentioned previously, the inverse of the loss rate need only be of the order of the pulse length for quenching from level 3 to occur. k, is certainly much larger than this minimum rate (~10’ s-‘) but not so large that ionization (rate u 5YI) does not compete at higher intensities. The above observations suggest that at intensities where quenching does not take place, as indicated by an 1’ fluorescence dependence, the population of level 3 is small. At the quenching threshold the probability of the third photon absorption during the laser pulse approaches unity, i.e. cr$yZT= I. The intensity at this point is roughly approximated as =lO*’ photons/cm’ s which coupled with the laser pulse length of NO-’ s gives a value for o&I as ~1O-l~ cm7. Although uncertain within at least an order of magnitude, this is a reasonabIe one-photon singletsinglet absorption cross section. If a similar value is assumed for o&l, then k, can be estimated at the point where ionization effectively competes with other quenching rates. This is the highest intensity region (I= 1O28 photons/cm* s) which upon substitution into k, w c@Tgives k, w 1011 s-l. If at this intensity 10% of the excited population is ionized, the total excited population, Ngti, is roughly ten times the measured ion current. Assuming a collection efficiency of unity, which is reasonable for the bias voltage and ion collection geometry used, this gives NY = 5 X lo8 moleciiles excited (per laser pulse). S~~,tot~=N&#~l*Z’~tiO being the initial number of molecules in the focal region, which for a 5 cm focusing lens is ~5 X lOI molecules), ~‘021can be estimated as =10-50 cm4 s. It must be emphasized that these values for #, kL and @, while reasonable,
and ionization potentials
of DABCO
are rough estimates, and can only be taken to lend support to the proposed dynamics. The overall picture of DABCO MPI dynamics can now be summarized. Fluorescence quenching which takes place at all but the Iowest intensities arises from further absorption from level 2 (the S, state) to a dissociative level (level 3). At high intensities, ionization from level 3 begins to overtake the other losses and becomes the major process up to total volume saturation. The ion current is thus Iimited by competition, in the third photon resonant level, from dissociation or other relaxation processes which remove the molecule from ionization or fluorescence. Such competition in the level(s) preceding the ion continuum, even in the two-photon state in a threephoton ionization, is likely to limit in any molecule the type of two-photon resonances detectable by single-beam MPI spectroscopy to long-lived states whose subsequent levels (if any) are also reasonably stable. This probably results in the particular sensitivity of the technique to molecular “Rydberg-type” states [4], whose generally sharp structure indicates little mixing with dissociative valence levels. By bypassing the dissociative intermediate levels (e.g. level 3 in fig. 5) with a W laser with one-photon excited state photoionization, the.spectroscopicaIIy interesting, low-lying (and longer-lived) “valence” states should thus become accessible. One further point can be made concerning DABCO MPI dynamics. As mentioned previously, the polarization ratio, a, which is determined by the symmetry of the two-photon transition can be measured directly through the ratio of/fluorescence arising from a circularly polarized laser over a linearly polarized laser. A one-photon transition does not have a polarization dependence for unoriented (gas-phase) molecules, i.e. !$I = @/& = 1. If the third and/or fourth photon absorption in the single-laser DABCO experiment takes place so fast, however, that the molecules excited by the initial two-photon transition still retain their preferred orientation with respect to the laser field, a non-unity S$) will be observed. The same overall !G? was found within experimental error for the ratio of the DABCO ion current as for the fluorescence, i.e. n MPI = &ror = I/40 (at a pressure of 0.5 Torr). Thus even though the ionization step follows an ~1’ dependence (two-photon ionization from the S1 state) the overall ionization process is biphotonic in charac-
D.H. Parker, MA. El-Sayed/Lifetimes and ionization potentials of DABCO
ter, and slower than the rotational disorientation
rate, for the intensity range of the experiment. It must be expected that at higher intensities or lower pressures than those used in this experiment ionization should overtake rotational relaxation. This observation will be complicated, however, by volume saturation.
4. Summary
The dual-beam MPI technique has been shown to be useful in measuring the S, lifetime and ionization potential of DABCO in a very simple way. This method which requires neither fluorescence nor vacuum W equipment should be applicable to a wide range of molecules. A qualitative picture of DABCO MPI dynamics has also been constructed by comparison of fluorescence and ionization. Single-beam MPI spectroscopy is seen to be limited by competition between ionization and dissociation or other quenching processes in the levels preceding the ion continuum. Such limitations should be overcome by the dual-beam MPI technique.
&knowledgement The authors would like to thank the National Science Foundation for their financial support.
References [l] P. Lambropoulos, Advan. At. Mol. Phys. 12 (1976) 87.
387
[2] P.M. Johnson, M.R. Berman and D. Zakheim, J. Chem. Phys. 62 (1975) 2500. [3] G. Petty, C. Tai and F.W. DaIby, Phys. Rev. Letters 34 (1975) 1207. 141 D.H. Parker, J.O. Berg and M.A. El-Sayed, in: Advances in laser chemistry, ed. A.H. Zewail, Springer Series in Chemical Physics (Springer, Berlin, 1978) p. 319. [S] S.V. Andreyev, V.S. Antonov, LN. Knyazev and V.S. Letokhov, Chem. Phys. Letters 45 (1977) 166. [6] C.S. James, I. Itzkan, CT. Pike, R.H. Levy and L. Levin, IEEE J. Quantum Electron. QE12 (1976) 111. [7] R.W. So&s, C.A. May, L.R. Carlson, E.F. Worden, S.A. Johnson, J.A. Paisner and L.J. Raidzieemski, Phys. Rev. A 14 (1976) 1129. [8] M. Robin, Higher excited states of polyatomic molecules, Vols. 1,2 (Academic Press, New York, 1974, 1975). [9] D.H. Parker and P. Avouris, Chem. Phys. Letters 53 (1978) 515. [lo] D-H. Parker and P. Avouris, in preparation. [ 1 l] P. Bischof, J.A. HashmalI, E. HeiIbronner and V. Homung, Tetrahedron Letters 46 (1969) 4025. [I2 ] A.M. Halpern, J.L. Roebber and K. Weiss, J. Chem. Phys. 49 (1968) 1348. [13] J.O. Berg, D.H. Parker and M.A. El-Sayed, J. Chem. Phys. 68 (1978) 5661. [14] A.M. H&em, Chem. Phys. Letters 6 (1970) 296. [15] E. Heilbronner and K.A. Muskat, J. Am. Chem. Sot. 92 (1970) 3818. 1161 A.E. Siegman, An introduction to lasers and masers (McGraw-Hill, New York, 1971). [17] S. Speiser and J. Jortner, Chem. Phys. Letters 44 (1976) 399. [18] D.J. Bradley, M.H.R. Hutchinson, H. Koetser, T. Morrow, G.H.C. New andM.S. Petty,Proc. Roy. Sot. London A 328 (1972) 97; D-T.Bradley, M.H.R. Hutchinson and H. Koetser, Proc. Roy. Sot. London A 329 (1972) 105.