Two-color resonant four-wave mixing spectroscopy of ammonia

Chemical Physics ELSEVIER

Chemical Physics 201 (1995) 237-244

Two-color resonant four-wave mixing spectroscopy of ammonia M.N.R. Ashfold a,1, D.W. Chandler a, C.C. Hayden a,*, R.I. McKay h,2, A.J.R. Heck b Combustion Research Facility, Sandia National Laboratories, Livermore, CA 94551-0969, USA b Department of Chemistry, Stanford University, Stanford, CA 94305-5080, USA Received 17 October 1994; in final form 27 July 1995

Abstract

We demonstrate the use of two-color resonant four-wave mixing (TC-RFWM) spectroscopy to detect gas-phase ammonia molecules. Appropriate choice of the two different excitation frequencies is shown to cause substantial enhancement in the third-order susceptibility, X(3), and a consequent increase both in spectral sensitivity and in selectivity as compared with the corresponding one-color degenerate four-wave mixing (DFWM) technique.

1. Introduction

The advent of multiphoton excitation methods has led to major advances in our understanding of the vertical electronic spectrum of the ammonia molecule [1-8]. All of the documented Rydberg states of ammonia have planar equilibrium geometries, whereas the ground state is pyramidal (due to the doubly occupied lone-pair orbital on the nitrogen atom). The first excited singlet state of NH 3 (the ~1,~/"2 state) is predissociated to the extent that the A - X absorption spectrum exhibits a vibronic progression in v[, the out-of-plane bending mode, but no resolved rotational fine structure [9-11]. Predissociation lifetimes of individual rovibronic levels of the ,~ state of ammonia have been estimated from

* Corresponding author. i Permanent address: School of Chemistry, University of Bristol, Bristol BS8 ITS, UK. 2 Current address: Department of Chemistry, Baker Laboratory, Cornell University, Ithaca, NY 14853, USA.

band-contour simulations [ 12], from analysis of resonance Raman spectra [13,14] and, most directly, through analysis of folded optical-optical doubleresonance spectra [15,16] proceeding via the higher lying, but much less strongly predissociated, (~' electronic state. Subsequent ab initio calculations of the multidimensional potential-energy surface of the .~ state of ammonia [17,18] and studies of the detailed energy disposal in the resulting NH 2 photofragments [19-21] have led to a fairly detailed understanding of the dynamics of this particular predissociation process. Two higher lying excited singlet states of ammonia, the B JE" and (~' Ja/~ states, have also been subject to detailed spectroscopic study. Compared with the ,~ state, both are very resistant to predissociation. As in the ,~-X system, the B state also appears in one-photon absorption [22] as a long progression in the excited state out-of-plane bending mode; however, a full interpretation of the detailed rotational structure accompanying each of these bands had to await the availability of (i) jet-cooled, and (ii) sub-Doppler, two-photon excitation spectra [7]. In

0301-0104/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0 3 0 1 - 0 1 0 4 ( 9 5 ) 0 0 2 6 9 - 3

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contrast, the (~' state was unknown prior to its observation, as a series of three-photon resonances, in the multiphoton ionization (MPI) studies of Colson and coworkers [1]. Subsequent two- and threephoton excitation studies of ammonia have led to refined spectroscopic constants for the various 2" levels of the C' state, and provided some insight into their predissociation dynamics [3,4]. Franck-Condon considerations dictate that any one-color excitation spectrum from the ground state to either the B or C' Rydberg states must be dominated by a progression in the out-of-plane bending mode. To learn about or (~' levels involving excitation of quanta other than v 2 it is helpful to turn to double-resonance methods. Several such studies, involving two-color double-resonant excitation via selected levels of the ground (X) a n d / o r the first excited (,~) state have been reported [8,23,24]. In this work we illustrate the use of a novel variant of four-wave mixing spectroscopy [25-28] to probe excited rovibronic levels of the B and C' states of NH 3. The present study represents a logical extension of the recent one-color two-photon degenerate four-wave mixing (DFWM) study of ammonia by Georgiev and Aldrn [29] in as much that, by using two different laser frequencies ( t q and to2), chosen so as to afford the maximum degree of resonance enhancement to the third-order susceptibility tensor

(a)

X (3), much improved sensitivity can be achieved. Further, as we show below, judicious choice of the frequencies to~ and to2 so that to~ is near-resonant with one of the predissociated 2~ bands of the ,~-,~ system can lead to considerable simplification of the overlapping and congested progressions associated with the B - X and C ' - X transitions.

2.

Experimental

Fig. l a shows a schematic of the experimental setup, whilst Fig. lb illustrates the excitation scheme and some of the vibronic levels of NH 3 relevant to the present study. The frequency-doubled fundamental of an injection-seeded Q-switched Nd: YAG laser (Spectra-Physics, GCR-5), operating at 30 Hz, was used to pump two dye lasers (Spectra-Physics, PDL2), by employing an 80% beamsplittero The output of the dye laser (DCM dye) pumped by the stronger of these beams was frequency tripled using KDP and /3 BO crystals (Inrad) to yield horizontally polarized UV radiation ~o~ in the wavelength range 210-220 nm. Various red dyes were used in the second dye laser to generate vertically polarized, tunable radiation at the required to2 frequencies (corresponding to wavelengths in the range 550-680 nm). This visible beam was split into two beams of similar intensities,

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Fig. 1. (a) Schematic of the experimental arrangement. (b) Energy level diagram for NH 3, showing the two-color resonance four-wave mixing scheme employed, and some of the vibronic levels relevant to this present study.

M.N.R. Ashfold et al./ Chemical Physics 201 (1995) 237-244

239

beam crossed through the focus of the red beams at the optimum phase matching angle of ca. 2 ° . Scattered light from the various optical components can be a major limitation in experiments of this kind; thus a number of irises were used to define the beam path of the four-wave mixing signal, here generated in the forward direction, after which it was turned through ca. 90 ° using a mirror (coated so as to provide optimal reflectivity at 220 nm and held on a precision kinematic mount), focused with an 80 mm f.1. lens through a spatial filter, then viewed through an appropriate interference filter by a solar-blind photomultiplier tube. Checks were made to ensure the observed signals depended on the presence of all three input beams and their mutual overlap in space and time.

focused with a ca. 70 mm focal-length lens and crossed at a full angle (20) of = 12° at their focus, in the middle of an evacuable cell containing ammonia gas (typical sample pressure 5 - 3 0 Torr). Considerable care was taken to ensure the difference in the path lengths traversed by these two beams was well within the pulse coherence length (estimated to be ca. 20 mm [30]). Wavelength calibration of the to2 beams was achieved by directing a portion of one of these beams, after passage through the cell, into an optoacoustic cell containing NH 3 gas (ca. 300 Torr) so as to record a part of its well documented AVN_H = 4 stretching overtone spectrum [31]. Alternatively, this to2 beam could be monitored using a photodiode so as to investigate the intensity dependence of the four-wave mixing signal. The optics steering the UV (to t) beam were arranged so this pulse also reached the centre of the cell synchronous with the two red (to2) pulses. All three pulses propagated in the same direction and in the same horizontal plane; the divergence of the UV beam and the exact position where it was incident on the focusing lens were chosen so the focus of the UV

3. Results and discussion

Fig. 2a shows part of the two-color four-wave mixing spectrum obtained using a static 30 Torr sample of NH 3 with to1 fixed at 46717 cm -1 (i.e.

(a) E x p e r i m e n t a l

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Two.Photon E n e r g y (cm "1) Fig. 2. (a) TC-RFWM excitation spectrum of a 30 Torr NH 3 sample obtained with coI = 46717 c m - 1 (A I = 214.0 nm) and to2 scanned in the wavenumber range 15500-15 150 cm -I . (b) Simulation of the corresponding part of the NH 3 (B-X) 23 two-photon absorption spectrum. This simulation employs ground- and excited-state rotational constants taken from the literature [6,7,32] and assumes that the transition is carried solely by the T~(A) component of the two-photon transition tensor, that the sample temperature is 300 K and that each rovibronic transition can be represented by a gaussian lineshape of 1 c m - ~ (fwhm).

240

M.N.R. Ashfold et al. / Chemical Physics 201 (1995) 237-244 (a) 46 007 cm" (b) 46 487 cm"j (c) 46 587 cm"l (d) 46 687 cm"s (e) 46 787 cm"t

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Energy (cm") Fig. 3. Calculated f o r m o f the N H 3 (,g,-X) 0 ° and 21~ one-photon absorption bands together with arrows indicating the ~oI frequencies used w h e n recording the various T C - R E - F W M spectra displayed in Fig. 4. The band contours were derived using spectroscopic constants for the g r o u n d a n d excited states taken f r o m the literature [15,32] and previously m e a s u r e d values for the predissociation-broadened linewidths of the 0 ° a n d 21 levels of the ,~ state [15,16]; the relative intensities o f the two bands were determined by c o m p a r i s o n with the published r o o m temperature absorption spectrum of N H 3 [ 14].

is worth considering what might be the possible benefits of using two colors. One obvious advantage, which emerges clearly upon comparison of the present results with the earlier one-color two-photon DFWM study of Georgiev and Alden [29], is sensitivity. Georgiev and Alden reported 100 mbar (76 Torr) as the minimum at which they could obtain results, whereas in the present work reliable spectra are already obtained at sample pressures as low as 5 Torr, indicating an improvement of an order of magnitude in sensitivity. Further, our pulse energies (typically < 75 IxJ for oJ~, = 750 IxJ for each of the a) 2 beams) are each one or more orders of magnitude smaller than those used in the earlier one-color study. Higher laser intensities produced broadened linewidths in the spectra indicating saturation of the transitions a n d / o r ionization of the sample. Care was taken to keep the spectral features laser linewidth limited. The reason for the dramatic improvement in sensitivity is clear. In the present two-color experiment, the signal-beam intensity Isig will be given by [33]

Is~g cx l X(3)lZl, 1212, on the long wavelength side of the A - X 2~ band see Fig. 3) and monitoring the UV signal beam whilst the visible laser was scanned over the wavelength range 660-645 nm (oJ 2 = 15150-15500 cm-J). The combined w I + w 2 wavenumber is indicated on the horizontal axis. The measured spectrum spans part of the known B - X 203 band of NH 3. The accompanying simulation (Fig. 2b) shows the expected form of the B - X 20 two-photon absorption spectrum, calculated using the appropriate Hamiltonian and spectroscopic parameters [6,7,32] and assuming that the transition is carried exclusively by the TZ(A) component of the two-photon transition operator, that the sample temperature is 300 K and that each rovibronic transition can be represented by a gaussian lineshape of 1 c m - 1 (fwhm). Comparison indicates that each of the features evident in the experimental spectrum is interpretable in terms of known two-photon transitions associated with this band. The obvious interpretation of this observation is that we are inducing a two-color four-wave mixing process. As the use of two colors (rather than one) inevitably makes for a more complex experiment, it

(1)

where, because we have a near one-photon resonance at w 1 but not at oJ2, the dominant contribution to X°)( oJs;w 1, w 2, - oJ2) will derive from the term X (3) (X E i

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Here g represents the initial state (X,0°), f the state resonant at the frequency ~oj + oJ2 (i.e. the B23 state in this case), and i the intermediate state(s) causing resonance enhancement at the one (oJ I) photon energy. In the particular case of interest here, oJ1 is near-resonant with the predissociated ,~-X 2~ transition - see Fig. 3 - and thus all three terms in the denominator in Eq. (2) tend to zero. Contrast this with the earlier one-color work [29], in which the first and third terms in the energy denominator would correspond to detunings >_ 15 000 cm - j . It is worth noting that this technique does not rely

M.N.R. Ashfold et al./ Chemical Physics 201 (1995) 237-244

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T w o P h o t o n Energy (cm "1)

Fig. 4. TC-RFWM spectra of 30 Torr NH 3 samples in the wavenumber range to I + to 2 = 6 3 7 5 0 - 6 4 1 0 0 c m - t for five different values of tol: (a) 46007 cm -1 (A I =217.30 nm), (h) 46487 cm -I (A 1 =215.05 nm), (c) 46587 cm - l (A I = 2 1 4 . 5 9 rim), (d) 46687 cm -I (A I = 214.13 nm), (e) 46787 cm -1 (A I = 213.68 nm).

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Fig. 5. Detail of the TC-RFWM spectra shown in Figs. 4a and 4e, together with calculated one-color two-photon excitation spectra of, respectively, the NH 3 ((~'-,'~) 0 ° and (B-,'K) 2~ bands. Spectroscopic parameters for the respective ground and excited states are taken from the literature [4,7,32], as are the relative weightings of the zero- and second-rank contributions to the (~'-X two-photon transition moment. Both simulations assume a sample temperature of 300 K and a spectral linewidth (gaussian) of 1 c m - i (fwhm).

242

M.N.R. Ashfold et al./ Chemical Physics 201 (1995) 237-244

upon the formation of a stationary population grating to obtain a single beam, but is a resonantly enhanced four-wave mixing process. The crossed beams, at to 2, are not one-photon resonant in the sample. Therefore, this experimental configuration should minimize thermal grating contributions to the signal. This can be contrasted to other recent gas phase four-wave mixing experiments [34]. Due to this insensitivity to thermal gratings this approach should be applicable to the measurement of two-photon spectra at high pressures. In the case of NH 3 this resonant two-color variant of FWM spectroscopy offers the further advantages of spectral simplification and selectivity. This is illustrated by the representative family of two-color resonant four-wave mixing (TC-RFWM) spectra displayed in Fig. 4. Each of these spectra was obtained by selecting a different (fixed) to 1 frequency and then scanning the wavelength of the second (to 2) laser over the appropriate part of the 550-600 nm range so as, in each case, to span the same (63 75064100 cm - l ) spread of two-photon wavenumbers. For orientation, Fig. 3 shows a simulation of the NH 3 absorption for a frequency range spanning the values of to l used here. Clearly, the fine structure observed in the TC-RFWM spectra varies sensitively with the chosen value of w I. To understand this dramatic evolution in the appearance of the two-color resonant four-wave mixing spectra we begin by noting that the one-color twoand three-photon excitation spectra of NH 3 at these same sum frequencies show two overlapping features associated with the C ' - X 00° and B-,'~ 205 bands [1,2,4,7]. Both of these bands also contribute to the TC-RFWM spectra shown in Fig. 4 but, by a judicious choice of to~, it is possible to selectively extinguish either band. Thus, for example, the spectra recorded with to~ = 4 6 0 0 7 cm -l and 46487 c m - i are dominated by the Q-branch associated with the C ' - X origin band, but as toj is increased, the more complex pattern of rotational lines associated with the B-.'~ 25 band starts to dominate. This is demonstrated more thoroughly in Fig. 5, which compares the TC-RFWM spectra obtained using w~ = 46007 cm -l (A l =217.30 nm) and 46787 cm -1 (A~ = 213.68 nm) with appropriate simulations of the two-photon excitation spectra of the C ' - X origin band and of the B-,X 25 band, respectively. We

emphasize that both these simulations (Figs. 5B and 5D), and that of the B-,X 203 band (Fig. 2B) are calculated two-photon absorption spectra, without inclusion of any specific resonance enhancements at the one-photon energy. They show the origins of the spectral lines observed in the TC-RFWM spectra. These simplified simulations match the observed spectra remarkable well. A detailed interpretation of the observed line intensities will require much additional investigation, both experimental (including systematic variation of the wavelengths and intensities of the to I and to2 beams) and theoretical (proper inclusion of the near resonant intermediate state, i, and calculation of X °), rather than the simple twophoton linestrengths. The vibronic selectivity found experimentally can be understood by returning to the expression for X ~3) (Eq. (2)) and noting in the case of NH 3, because of symmetry considerations, the ( i l / ~ l f ) and ( f l / x l i ) matrix elements will be nonzero only in the case that Av 2 is even. Thus the C ' - X origin transition will be much enhanced by tuning to 1 into near-resonance with the .~ 0 ° level, whilst the B-~( 205 transition will be favoured by use of o.)1 frequencies in near-resonance with the ,~ 21 level. Note that none of the TC-RFWM spectra shown in Fig. 4 were recorded with toj set close to the A - X 2 0 band centres: attempts to record such spectra were hampered by absorption of both the wj input and signal beams. Returning to the spectra shown in Fig. 5, it is clear the simulations succeed in reproducing the positions, though not the relative intensities, of all of the stronger rovibronic features observed experimentally. Several factors contribute to the discrepancies between the experimental and calculated line intensities. One is a simple operational factor: none of the experimental spectra have been corrected for the way the pulse energy of the to2 laser varies with wavelength; this accounts for the absence of weaker t~'-X features at high wavenumber in Fig. 4a. Other reasons arise because the experimental and calculated spectra are not directly comparable. The simulations show calculated one-color two-photon excitation spectra, assuming no level specific resonance enhancements at the one-photon energy. Yet recalling Fig. 3, and notwithstanding the predissociated nature of the ,g,-state levels, it is clear that at any particular to I frequency some A-state levels will be closer to --

n

M.N.R. Ashfold et al. / Chemical Physics 201 (1995) 237-244

resonance than others; two-photon transitions proceeding via such levels will inevitably appear with enhanced intensity. Indeed, this is the rationale (at the purely vibronic level) for the observation that the T C - R F W M technique provides a ready means of distinguishing between overlapping B-,X and C ' - X resonances. Further, we must recognise that the intensifies of individual ro vibronic lines in the calculated one-color two-photon excitation spectra scale with the a p p r o p r i a t e t w o - p h o t o n rotational linestrengths whilst the experimental line intensities, arising via a four-wave mixing process, will scale with the square of these linestrengths. Thus, as in all four-wave mixing experiments, the observed spectra tend to be dominated by the stronger transitions - an effect which will tend to be moderated, in part, by any saturation effects. No attempt has been made here to quantify the effects of saturation, only to eliminate the obvious line broadening due to saturation and ionization at higher laser intensities.

4. Conclusion We report one of the first applications of two-color resonant four-wave mixing spectroscopy to a gasphase polyatomic molecule, viz. ammonia. The present results serve to highlight (a) the increased sensitivity and (b) the spectral selectivity that can result given an appropriate choice of excitation frequencies. With some additional work this technique has the potential to be an important tool for the study of two-photon spectroscopy and for quantitative population determinations.

Acknowledgements MNRA is grateful to the Nuffield Foundation for a travel grant, and to Drs. I.R. Lambert and C.M. Western for helpful discussions. RMcK acknowledges support from the National Science Foundation. AJRH thanks the Netherlands Foundation for Scientific Research (Nederlandse Organisatie voor Wetenschappelijk Onderzoek) for a N W O / N A T O fellowship. This work is supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences.

243

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