Resonance-enhanced multiphoton ionization of vibrationally excited state-selected NH3. Spectroscopy and collisional state transfer

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CHEMICAL PHYSICS LETTERS

29 April 1988

RESONANCE-ENHANCED MULTIPHOTON IONIZATION OF VIBBATIONALLY EXCITED STATE-SELECTED NHS. SPECTROSCOPY AND COLLISIONAL STATE TRANSFER Thomas SEELEMANN, Peter ANDRESEN Max-Planck-Institut fiir Striimungsforschung, Bunsenstrasse 10, D-3400 Giittingen, Federal Republic of Germany

and Erhard W. ROTHE ’ Research Institute for Engineering Sciences and Department Detroit, MI 48202, USA

of ChemicalEngineering, Wayne State University,

Received 25 September 1987; in final form 2 February 1988

Infrared laser light prepares g-state NH, in selected JK levels of the v,+v, combination vibration. Ultraviolet laser light performs 2+ 1 REMPI on those states. This is used to detect those states and thereby carry out state-to-state collision experiments, and to find previously unobserved vibrational levels (i.e. v, and v,+v,) of the B Rydberg state.

1. Introduction There are several recent studies of resonance-enhanced multiphoton ionization (REMPI) of ground state NH, via its Rydberg states. An excellent review is available [ 11. Ammonia is pyramidal in the ground (x) state because it has a pair of non-bonded 3a, electrons on the N atom. Its Rydberg states, which result from the promotion of a 3a, electron, are all planar. The Franck-Condon principle shows that such a transition yields vibrational excitation in the symmetric out-of-plane bending mode ( yz) and REMPI studies have observed and analyzed such v2 progressions. Room-temperature IR and REMPI spectra are complex because many transitions can occur. Here we describe a double-resonance experiment in which single states are prepared with IR light and detected with REMPI. Analogous experiments with NO have been reported [ 2 1. Advantages of such experiments are (a) better identification of IR transitions when the REMPI detection is tuned to probe a single state, ’ Guest at MPI, where the experiments were conducted.

and conversely, (b ) improved analysis of a REMPI spectrum when the IR prepares a single state. Further, excitation of different vibrational modes in the electronic ground (z) state may lead to new vibrational modes in the Rydberg states. Finally, with the use if suitable delay times between the IR and UV lasers, the state-specific REMPI probe can monitor state-to-state kinetics.

2. Apparatus Coaxial antiparallel IR and UV laser beams are for cused (by 25 cm focal length lenses) into the same small volume inside a cell containing flowing NH3 (or gas mixtures). The REMPI current is collected on parallel plates and is sent to a boxcar. The cell pressures are measured with a capacitance manometer and are in the range 0.1-0.3 Torr. A beam splitter sends some IR light into another NH3 cell (at 20 Torr) that has a microphone and so an absorption spectrum can be recorded via the photoacoustic eifeet. The REMPI and photoacoustic signals are written on a two-channel strip chart recorder.

0 009-26 14/88/$ 03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division )

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Tunable IR is produced by Raman-shifting light (third Stokes) from a YAG-pumped dye laser (with R6G) into the 2 pm region inside a Hz-filled ( z 25 atm) cell and it has ~0.2 mJ/pulse energy at ~0.2 cm- ’bandwidth. This IR excites NH3 to the v,+v, combination vibration of the R state, i.e. the z(O, 0, 1, 1) state, where v3 and v4 are degenerate asymmetric bending and stretching modes, respectively. These prepared states are subjected to (2+ 1) REMPI with UV light in the range 335-355 nm. The UV comes from an excimer-pumped dye laser (with PTP or DMQ) that as = 15 mJ/pulse at x0.25 cm-‘. The pulse widths were x 10 ns. The UV is adjusted to come 15-300 ns after the IR. The UV and IR have mutually perpendicular linear polarization. The UV wavelengths are calibrated, via the optogalvanic effect, against rive neon lines in the range 346.7-352.1 nm and are accurate to + 0.01 nm there. Vacuum wavelengths are used in this Letter. We want REMPI currents ZiRUvcaused by double resonances, Fig. 1 shows that some unwanted REMPI currents Z,, also occur without IR and so we usually use the following scheme. The IR beam is at 5 Hz and the UV at 10 Hz. When both beams are fired the signal is ZIRUV+ZUv. When only the UV is on, the signal is Zuv. The boxcar can subtract Z,v from ZrRuv+ Zvv, as shown in fig. 1. The subtraction elim-

29 April 1988

inates Zvv, although the laser’s energy fluctuations result in a noisier baseline.

3. Experimental prucedure The IR spectra from the photoacoustic cell are recorded and compared with Sara&s [ 31 absorption spectrum. He has identified many of the transitions and so we can directly apply his assignments. Two types of double-resonance spectra are measured: either the UV is scanned at a fixed IR frequency or the reverse. For the former, we select resolved IR lines that (a) are reasonably strong and are identified by Sarangi, and (b) yield a good REMPI spectrum. (Most IR lines produce no REMPI signals in our UV wavelength range.) Some fraction of the prepared states are destroyed by collisions. Consider the data in fig. 2. The UV laser frequency is fixed at 339.68 nm so as to detect one specific rotational level in the R (0,0, 1, 1) state. The IR laser is then scanned, and photoacoustic and REMPI signals are recorded. Panel (a) shows a pure IR spectrum from the photoacoustic cell. Panels (b ) , (c), and (d) are corresponding REMPI spectra as a function of system pressure P, delay time t, and collision partner. Panel (b) shows six lines. Panel (c), which is for greater P and t, shows that most of those six lines are smaller but also that new lines appear. the intensities of the primary peaks decrease because many of the molecules undergo state-changing collisions, the probability of which is proportional to Pt. Conversely a portion of other IR prepared states convert into the detected state and so they now appear in the REMPI spectrum. Panel (d) has similar spectra from loo/oNHs, 90% H20.

4. Analysis 4.1. Collisional transfer

Fig. 1. REMPI signals with (lower trace) and without (upper trace) background subtraction. The upper trace shows an undesired signal 1,” (the B(O, 0, 0, 0)-x(0, 1, 0, 0) hot band) in addition to the desired IR-UV double resonance 1,,,,. The lower trace has only 1,,“,.

90

Assignment of the IR transitions is reasonably simple with the aid of Sarangi’s table. The two strongest lines in fig. 2b (at 4967 and 5106 cm-’ ) can be assigned to 6, - 5e 6,6 (Sarangi’s line numbers No. 1233 and No. 1236 at 5106 cm-‘) and 6, -5+ 7,6 (No. 555 and No. 558 at 4967 cm-‘).

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Volume 146, number 1,2

6.+5

6.- 5

a1

0

[cm-‘)

6,-S

b,

6,-S ?,-5

Cl

6,-S

6.-5 7,-5

dl

Fig. 2. Panel (a) shows IR photoacoustic signals. Panels (b), (c), and (d) compare REMPI signals at different pressures P, delay times t, and collision partners. The W detects 6, - 5. At greater P and t, other prepared S, *K’ collisionally transfer into 6, - 5 and are detected. Panels (b), (c), and (d) have P= 100, 150, and 300 mTorr and k20, 150 and 120 ns, respectively, Panel (d) is for 10% NHr-90% H20, the others for pure NHs.

The rotational levels of this degenerate vibrational state are described as J, + K where the f refers to the relative direction of the’vibrational angular momentum and K. Because the a-s separation in the B state is large, we probably detect only one of the two inversion levels but our IR resolution is inadequate to tell which.

The lines at 5125 and 5005 cm-‘, marked with an. asterisk, occur near the wavelength where the level 6, +5 is populated. However they do not match as well with Sarangi’s assigned lines, and may be due to the underlying parallel band or to a hot band. They are also primary lines because they diminish with t. 91

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The weak lines at 4987 and 4947 cm-* are the IR transitions 5, - 5 +6,6 and 7, - 5 t8,6. These lines are detected because they have a large collisional conversion efficiency to 6, - 5. Panel (c) displays several secondary peaks that are caused by collisions that convert prepared rotational states to detected 6, - 5 states (note that not all IR prepared states convert to 6, - 5 ). Obviously the 5, -5 and 7, - 5 peaks became larger. The second largest 6, - 5 peak in panel (b) changes less. This is because the IR there also prepares 7, - 5, which then transfers to 6, - 5. Other secondary peaks, which are not resolved by J because of inadequate IR resolution, but are clustered by K, are so labeled, All the labeled lines, except those marked with an asterisk, probe states of symmetry E and belong to the ortho system of NH3. Obviously the symmetry of the prepared states is conserved in the collision. 4.2. State-to-state rate constants Fig. 2 also provides a basis for state-to-state rate constants within the v3+ v4band of the 2 state. More quantitative results are obtained by analyzing the intensities of REMPI signals as a function of collision time t. The large, primary, 6, - 5 peak at the right of panel (b) initially decreases exponentially with t. A secondary peak, e.g. when 5, - 5 is prepared and 6, - 5 is detected, increases to a maximum and then decreases. Examples of each are shown in fig. 3. The primary peak decay leads directly to the second-order rate constant ke5 for its collisional conversion into any other state. For simplicity, we also use pseudofirst-order rate constants kpJK=nk,,, where n is the gas density, but list values for second-order constants. The prime has been omitted from J and K. Then In(l/l,) = -k&, where the I are the REMPI signals, i.e. the IIRuv, which are proportional to the 6, -5 density [6, - 51. In pure NH3, kh5 is (8.0f2.5) x lo-lo cm3/molecule s. When 5, - 5, for example, is prepared by the IR, we have the pseudo-first-order scheme kvss (all J’, K’ except 5, - 5 ) -5,-5 kpss -6,-592

kp6s

(allJ’,K’ except 6, -5))

29 April 1988

Fig. 3. Signals proportional to the 6, - 5 density as a function of delay time t, at 0.11 Torr, (a) when 6, - 5 (triangles) is initially prepared, (b) when 5, -5 (squares) or 7, -5 (crosses) is prepared. The fits are calculated from eq. ( 1) and using the rate constants in table 1.

where the kplKis for state destruction and kpss is for the state-to-state transition. If [ 5, - 5 ] decreases exponentially with t (see below), then, from this mechanism, [6, - 51 can be shown to be related to the initial 5, -5 density [5, -510 by [6> -51={(~,,)[5>

-5lol(k,,,-k,,))

X[exp(-kp55t)-cxp(-kp65t)l.

(1)

A fit of the data to eq. ( 1) yields values for bss and bS5. In order to appreciate their dependence upon the data, consider two limiting cases. The maximuim for [6, -51, at tmax,is when d[6, -5]/dt=O (i.e. when dZ/dt ~0) and occurs when kp55cxp(-kp55tmax)=kp65 cxp(-kp65tmax),

(2)

and I&~ is known from the decay of the primary peak. only La, is then needed. However, he5, which is measured at shorter t, is more accurate than kpss because some particles drift out of the focus region and so the assumption of an exponential decrease of [ 5, - 5 ] becomes invalid. (Remember that we have a probe for 6, -5, but none for 5, -5 or 7, -5.) A second limit from eq. ( 1) is the slope at t =O, i.e. (d[6, -5]/dt)o=&ss[5, -510, and this re-

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CHEMICAL PHYSICSLETTERS

quires initial relative values for various [J’, +K’ ] 0. We measure Is5 and then relate the [ 5, -51, and [ 6, - 5 ] ,, by assuming that these are proportional to Sarangi’s [ 31 IR line intensities (&,)x. (Other estimates of these relative initial densities from (a) our own absorption measurements, (b) the 300 K Boltzmann distribution of lower states, or (c) from our photoacoustic signals, agree within 12046.) Let ( Z65)obe the signal when 6, - 5 is prepared and (d&,/dt), the initial slope when 5, -5 is prepared. Then

29April1988

quantum of v3, i.e. it is B(O, y, 1, 0) [ 1,4], and has Ev,=2785 cm-’ (compared with 3444 cm-’ in the ii state). It also has a progression in v2. Fig. 4 shows a REMPI spectrum originating from x( 0, 0, 1, 1) 2, f 1 prepared via Sarangi’s line No. 904. This spectrum is uncongested, but has enough lines to encourage analysis. The energy scale at the

(3) Accordingly the state-to-state rate constants k&s require relative initial densities of prepared states but the state destruction constants h,, and kpes do not. However kps5is less accurate because t is larger. These statements also apply to the fits to eq. ( 1). Table 1 shows early results for several second-order rate constants, each with various collision partners. 4.3. Vibrationally excited Rydberg states Of the many known electronic states [ 1,4] of NH3, only the B and c states are relevant here: the higher states are beyond our wavelength region. The normal B-state spectrum #’ has a progression in v2, i.e. B(O, u2.0,O) and it is often called B(v2) 161. The c state is actually the 8 electronic state that has one

62350

Table1 Valuesfor second-order rateconstantskJK for destructionof initial stateJ’K’of NH, ft(0, 0, 1, 1) and kJKds for state-to-state conversionof J’K’to 6, - 5 by variousmolecules.The error is estimatedto be k 30%for k5 andks5, and f 60%for k,& Units are lo-” cm3/molecule s. Mixturescontain 10%NH,, but exceptforpureNHs,therateconstantsignoretheeffectofNH,-NH, collisions

355

61420 Collision partner

k65

k55

k,5

Ls

k,,-,,

NH3 HXJ He Ar

8.0 5.3

15 19

24

5.0

I1

41 3.3

6.8 0.3

23 0.3

3.0

1.8

3.0

2.1

(r’See, e.g., fig, 2 in ref. [S].

0.2

REMPI

SIGNAL

Fig. 4. REMPI spectrum from IR-prepared state 2 la. The scale at the right is the UV wavelength (vacuum) while that at the left. is the Rydberg’senergy, i.e. theenergyofstate 2, + I (5112cm-‘) plus that of two UV photons. The band origins vooof various vibrational states B(0, uz,4, uq)are listed. Those with u4= 0 have known energies, and some JK assignments are indicated. Those with uq= 1are unknown, and the shaded areas represent a range of uqenergies between 1300 and 1600 cm*‘. 93

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left is the sum of the 2, + 1 state energy (5 112 cm-’ [ 3 ] ) and that of two UV photons. The expected position of various B state levels is also indicated; vq is unknown for this state so a range is given. Calculation of frequencies of allowed transitions from the 2, + 1 state to known B [4,7,8] or c [4,9] state levels do not yield any obvious matches to the data, and so these transitions must be to previously unobserved vibrational modes of the B state. The Franck-Condon principle combined with the selection rules seen in earlier REMPI work [ 10 ] suggests that a prepared %(0,0,1, 1) E state should still yield a B(O, Q, 1, 1) E or A series. However these states are energetically inaccessible within the UV wavelength range of fig. 4. Unfortunately rotational assignment of the spectrum was impossible. However with the aid of the energy scale, we attempt an analysis of possible B-state vibrations. Consider the sequence of lines at 349.35 and 354.56 nm, which we assume are transitions to 8( 0, 2, 0, 1) and fi (0, 1, 0, 1), respectively, on the following bases. These two bands are separated by 841 cm-’ from each other, which can be compared with 933 cm- #1for @O, 2, O,O)-~(O,l,O,O)orwith860cm-’ [4]“forB(O, 2, 1, 0)-B(0, 1, 1,O). These assignments also imply that the extra quantum of vq contributes 1307 and 1399 cm- ‘, for vz= 2 and 1, respectively, compared with 1626 cm-’ in the g state. A similar reduction in the v3 energy from the g to the i? state was noted above. Fig. 5 shows a similar REMPI spectrum with the I2For B (0, u2,0, 0), and u2= 0, 1, 2, 3, and 4, respectively, the constantsofEbataeta1. [7] areYw=59225, 60 121,61 054, 62016and63003cm-I. x3For B(0, 0, 1,O) and B( 0, 1, 1, 0),respectively, the constants (incm-‘)ofDouglas [9] arevw=62010,62870;B,=10.18, 9.67, and,C,=4.97, 5.00

*3

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IR set at Sarangi’s line No. 555 or No. 558 to prepare 6, - 5. The wavelengths are shorter than in fig. 4 (it includes the 339.68 nm line used in figs. 2 and 3) and so can reach B(0, 0, 1, 1). For each g(O, 0, 1, 1) E’ level prepared we might expect to see a set of UV resonances to a fixed K and various J (with Ja K) and separated by 285. Such a set is seen in fig. 5, which is consistent with 7, 6-6, -5-+J, 4 with the final level having E” symmetry. This gives B,= 10.55 f0.06 cm-‘. Comparable values are B,= 10.59 [7] and 10.18 cm-’ [9] for B(O, 0,0,0,),and B(O, 0,1,O) , respectively. Unfortunately we could not find other analogous progressions. This suggests either considerable perturbation or strongly level-dependent predissociation such that only a few final-state levels are sharp. We thus cannot determine other spectrosopic constants for this state and we can only estimate the vibrational origin at 4780 cm-’ (to be comparcdwith5053crr-‘fortheR(O,O, 1,1) E’). This is in contrast to the previously identified levels of the B state, which show regular structure. This confirms that we are seeing new vibrational levels of the B state. Fig. 4 does not show any such simple progressions and so is consistent with the hypothesis of strong perturbation or predissociation. It is also possible that these arise from states produced by hot band excitation by the IR which will have favorable Franck-Condon factors here [ lo].

5. Discussion Double-resonance experiments permit research with single states in polyatomic molecules like NH3. Because those states can be selected they have an advantage over nozzle preparation. We show here that this can be used to do collision experiments with selected states. We also use this method to prepare Rydberg states with vibrations that have not been accessible by other means.

6

w t P

J=L 1

Acknowledgement

5 Jl

7 /w

Xl

3LO

339

1 tnml

Fig. 5. REMPI spectrum from 6, -5 preparation. Assignments correspond to the transitions in the B (0,0, 1, 1) state.

94

We are grateful to C. Western and M. Ashfold for helpful discussions. EWR thanks H. PauIy, Director at the MPI, for the opportunity to work at his institute and AFOSR and NSF for partial support.

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References

[l] M.N.R. Ashfold, C.L. Bennett and R.J. Stickland, Comments At. Mol. Phys. 19 (1987) 181.

[ 21 P. Esherick and R.J.M. Anderson, Chem. Phys. Letters 70 (1980) 621.

[ 3 ] S. Sarangi, J. Quant. Spectry. Radiative Transfer 18 ( 1977) 257,289. [4] J.H. Glownia, S.J. Riley, SD. Colson and G.C. Nieman, J. Chem. Phys. 73 (1980) 4296.

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[ 5 ] R.J.S. Morrison, W.E. Conaway, T. Ebata and R.N. Zare, J. Chem. Phys. 84 (1986) 5527. [ 61 G. Hetzbe&, Molecular spectra and molecular structure, Vol. 3. Electronic spectra and electronic structure of polyatomic molecules (Van Nostrand Reinhold, New York, 1966) p. 87. [ 71 T. Ebata, W.E. Conaway and R.N. Zare, private communication (1986). [ 81A.E. Douglas and J.M. Hollas, Can. J. Phys. 36 ( 1961) 479. [9] A.E. Douglas, DiscussionsFaraday Sot. 35 (1963) 158. [lO]M.N.R. Ashfold, R.N. Dixon, R.J. Stickland and CM. Western, Chem. Phys. Letters 138 (1987) 201.

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