Photoionization spectroscopy of LiMg

7 November 1997

CHEMICAL PHYSICS LETTERS ELSEVIER

Chemical Physics Letters 279 (1997) 44-49

Photoionization spectroscopy of LiMg K.R. Berry, M . A . D u n c a n * Department of Chemistry, Uniuersity of Georgia, Athens, GA 30602, USA Received 14 July 1997; in final form 15 August 1997

Abstract We report the first electronic spectroscopy for the metal diatomic LiMg. Two electronic band systems near 330 nm are measured with resonant photoionization spectroscopy. Both systems correlate to the 2s-3p (2p ~ 2S) transition of lithium. 11 bands are observed for the weak E ,---X and F ~ X transitions originating from the ~," = 0 ground state vibrational level. Vibronic hotbands reveal the ground state vibrational frequency (t%") of 190 cm-~, and extrapolation of the ground state levels yields the dissociation energy of D'~ = 1330 cm-a (0.165 eV; 3.8 kcal/mol). © 1997 Elsevier Science B.V.

1. Introduction Metal-containing diatomic molecules provide diverse examples of chemical bonding. Electronic spectroscopy of metal dimers produced and studied in the gas phase has provided detailed information on bond energies, bond distances and electronic configurations [1-3] for some species, but surprisingly, there are many simple diatomics for which there are no experimental data. On the other hand, the various diatomic metal species composed of light elements in the second and third row have been extensively investigated with theory [4-9]. Experimental measurements on mixed-metal dimers such as these are often limited by the lack of suitable alloy samples. However, we have recently developed new methods to produce composite samples containing immiscible elements which are used in a laser vaporization cluster source. In the present Letter, we apply these methods to obtain the first spectroscopy of LiMg. * Corresponding author.

LiMg is an interesting system for study because the corresponding homonuclear diatomics (Li 2 and Mg 2) are well known [3]. The three valence electrons of the Li and Mg atoms combine to form a covalent bond with a configuration of (scr)2(scr*) 1 and a bond order of 1/2. The bonding in the 2S~+ ground state is therefore expected to be relatively weak. The limited number of electrons in Li and Mg have allowed this system to be investigated with high-level ab initio calculations [4-9]. The bonding is calculated to be weak ( D o = 3-5 kcal/mol). However, in the only experimental measurement of the bond energy, a thermochemical study has reported D O= 15.2 + 2 kcal/mol [10], which is significantly out of line with theory. Except for unresolved emission spectra [6], there are no previous spectroscopic studies of this system. In the present work, we report the first observation of a vibrationally resolved electronic spectrum for LiMg, where two band systems are analyzed. From this, we determine the excited state properties as well as the ground state dissociation energy and vibrational frequency.

0009-2614/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. PII S 0 0 0 9 - 2 6 1 4 ( 9 7 ) 0 0 9 9 1 - 3

K.R. Berry, M.A. Duncan/Chemical Physics Letters 279 (1997) 44-49

45

2. Experimental Metal-containing species are produced in a molecular beam experiment with laser vaporization in a pulsed nozzle cluster source, using a double-solenoid valve (Newport BV-100 with home-made modifications). However, alloy samples containing the lithium and magnesium are not generally available for these studies. We have previously described methodology for the preparation of composite samples of LiAg and LiCu consisting of a thin film of lithium deposited on the surface of a solid rod of the other metal [ 11 ]. Composite samples of LiMg are prepared in this same way. A resistively heated alkali metal oven is employed to deposit a thin film of lithium on the surface of a magnesium rod (1/4" dia.). The film-coated rod is transferred from the deposition chamber to the molecular beam machine under an argon atmosphere to limit oxidation of the surface, and it is loaded and placed under vacuum immediately. A frequency-doubled Nd:YAG laser at 532 nm (Spectra-Physics GCR-I 1) is used to vaporize the sample. The intensity of this laser is adjusted to penetrate through the lithium film to the underlying magnesium, producing both atoms in the resulting plasma. Production of LiMg is particularly sensitive to the intensity of the vaporization laser ( < 30 m J/pulse) and the rotation speed of the sample. The metal-containing plasma is cooled in a pulsed supersonic expansion optimized for the production of small cluster species (i.e., using a short growth channel after the vaporization point). The molecular beam then enters the extraction region of the time-of-flight mass spectrometer, where photoionization occurs. The beam machine and mass spectrometer have been described previously [12]. Ionization is accomplished with a frequency doubled Nd:YAG pumped dye laser system (Spectra-Physics G C R - 1 5 0 + P D L - 2 ) with - 1.0 cm-~ resolution. Spectra are recorded in three mass channels (TLiZ4Mg, 7LiZSMg, and 7Li2~'Mg) to detect isotopic shifts. Spectra are calibrated with the 2 s - 3 p and 2s-3d atomic transitions of the Li atom [13].

3. Results and discussion Fig. 1 shows the molecular states resulting from the combination of lithium and magnesium atoms.

30.ooo-- ~

~(3p)(2P) \ ~ - - ~ /

20.000

+ Mg (tS)

l.i(-s)+Mg (~P-~-

E (cm -1 )

C.D

Io,ooo ~

~

+ Mg (IS)

A.B

0.0

-- ~ [ x2z+

~

~

1

Li ('S) + Mg ( S )

t

[

I

R Fig. l. Schematic energy level diagram showing the low-lying molecular states of LiMg and the atomic states of Li and Mg to which they correlate. The band systemsreported here are believed to correlate to the Li 2s-3p atomic transition.

The ground 2 ~ + state correlates to Li (2 S) + Mg( I S). A and B excited molecular states (2Z~, 211; order not known) are expected correlating to the Li(22p) + Mg(LS) asymptote, and excitation into these excited states might be expected near the corresponding atomic 2P ~ 2S transition at 14904 cm -1. C and D excited states (2~+, 2ii [6] order not known) are expected correlating to the L i ( 2 2 S ) + Mg(33p) asymptote. E and F excited states are expected correlating to the Li(32 P) + Mg( 1S) asymptote, and excitation into these states might be expected near the 2p ~ 2 S transition at 30925 cm ~. Higher states are also expected correlating to the L i ( 2 S ) + M g ( I P ) asymptote at 35051 cm-~ (not shown). The ionization potential of LiMg has been determined to be 5.96 eV [10]. Therefore, 1 + 1 photoionization via the A or B excited states as intermediates may not be possible with one-color excitation. However, onecolor 1 + 1 ionization should be possible via the E or F (or higher) excited states, and this is the region selected for this experiment. Excitation to the A or B excited states correlates to the intense 2p ~ 2S atomic transition in Li, which carries most of the atomic oscillator strength. Therefore, the absorption cross-

46

K.R. Berrv M.A. Duncan/Chemical Physics Letters 279 (1997) 44-49

E'e-X

F 4--- X

1

2

I

1

1 I

0

I

2 I

. m

Ill e-

em

6~

.>_ m

gO.

l

I

I

I

I

I

30500 30700 30900 31100 31300 31500 31700 31900 32100

Energy (cm "11 Fig. 2. E *-- X and F ~ X band system observed for 7Li24Mg near 320 nm.

LiMg

1,0

X 2 T.+--~ E 0,0 ,4,=e

¢oo (D ¢>

.m

(1)

2,1 1,1 ),1

1,2

2,3

I

I

30200

30300

30400

30500

30600

Energy (cm "1) Fig. 3. An expanded view of the E ~- X origin region of the spectrum showing hotband vibronic levels used to determine the ground state vibrational constants.

K.R. Berry, M.A. Duncan/Chemical Physics Letters 279 (1997) 44-49

section into the A or B states would likely be much greater than that for the E or F states. To search for the LiMg spectrum, we scanned initially with a dye-plus-doubled dye configuration at visible red wavelengths, which would allow detection of the A or B states via 1 + 1 absorption (o92 = 2 o9~; o9~ = red) or detection of the E or F states via 1 + 1 absorption ( 0 2 = ogl; o9~ = UV). Based on the quoted ionization potential, the A or B state position may or may not allow detection of these states via the dye-plus-doubled-dye configuration, but other two color schemes are presently not available on this instrument. Fig. 2 shows the photoionization spectrum measured in the LiMg parent ion channel as the laser is scanned near 660 nm. No signal is found resonant with the red visible photon, but a signal resonant with only the ultraviolet is found. We confirm that this is a ' U V only' signal using a filter to block the visible light. Throughout the 330 nm wavelength region, there is a series of sharp resonances with a noisy underlying continuum. However, the resonances are reproducible and these spectra are observed in the parent LiMg + mass channel for each of the three magnesium isotopes(7 Li 24Mg, 7Li 25Mg, 7Li2~Mg), confirming the identity of the molecule. Because of the location near the corresponding atomic resonance line (indicated with a vertical dashed line) we expect that this spectrum corresponds to excitation of the E a n d / o r F excited states. Consistent with the intensity of the corresponding atomic transition, the bands measured here are extremely weak in intensity. Due to the weak signals and the noisy continuum underlying the sharp bands, we have attempted additional scans with extremely long averaging at each wavelength step. This procedure has focused on the lower-energy region of the spectrum, where the continuum is somewhat weaker. Fig. 3 shows the spectrum in this lower-energy region taken under these high averaging conditions with much improved signal-to-noise. By consideration of band positions under both conditions, and the spectra measured for the three isotopomers, we are able to achieve a consistent assignment for essentially all the bands observed. We assign main progressions and associated hotband transitions for two electronic band systems. The lower system has an origin near 329 nm and the upper system has an origin near 321 nm. The vi-

47

Table 1 Band positions of the E '-- X transition of 7 Li24 Mg t l tt U ,

E

(cm r) 0. I 1,2 2, 3 0,0 1, I 2, 2 1,0 2, I 2, (1 3, I 3,0 4,0 5,0

30196.1 30264.5 30338.2 30370.1 30425.5 30486.0 30597.1 30646.4 30831.4 (30860.2) 31054.9 (31284.3) (31511.6)

Bands in parentheses are not included in fits to determine constants because of perturbations a n d / o r poorly determined positions.

bronic assignments are indicated in Figs. 2 and 3, and the band positions observed in the 7Li24Mg mass channel are given in Table 1. The isotope shift observed for these bands is quite small, even for the higher vibrational levels, and it is irregular as a function of c. An unequivocal numbering of the vibrational levels is therefore not possible. However, the first band in each of the two band systems exhibits essentially zero isotope shift and there is a sudden onset in intensity, so we are confident that the origins are assigned correctly. Two excited states are expected to lie in this region correlating to the Li(32 P) + Mg( ~S) asymptote, and so we assign these excited states as E and F. Without rotational analysis, we cannot assign which of these states are the expected 2Z+ and 211. All the bands observed are noticeably blue shaded, indicating that the excited states have shorter bond distances than the ground state. However, while we attempted rotationally resolved scans, these experiments were unsuccessful due to the extremely low signal levels. Least-squares fits of the main progression members and their associated hotbands provides vibrational constants for these excited states and for the ground state. These are w'e = 228.9 cm-L and we x'e = 0.37 cm ~ for the excited E state, and og'e= 255.9 cm l and ogex'e = 4.64 c m - l for the excited F state.

K.R. Berry, M.A. Duncan / Chemical Physics Letters 279 (1997) 44-49

48

Table 2 Band positions of the F ,-- X transition of 7Li24Mg

Table 3 Vibrational constants for 7Li24Mg

U t , U tr

State

~0o (cm- i)

oJe (cm- i)

o)e xe (cm i )

Do (cm- t)

X 2~+ E F

0 30370.1 31160.6

190.0 228.9 255.9

6.79 0.37 4.64

1330 -

E

(cm-') 0,0 1,0 2,0 3,0 4,0

31160.6 31407.2 31644.6 (31883.3) (32121.8)

Bands in parentheses are not included in fits to determine constants because of perturbations a n d / o r poorly determined positions.

Using the hotbands, the ground state constants are 09e" = 190.0 cm -1 and o2ex'~=6.79 cm - j . These various spectroscopic constants are given in Table 2. In principle, the vibronic progressions can be extrapolated to obtain the dissociation energy in these excited states and to verify the atomic asymptotes to which these molecular states correlate. However, this procedure produces anomalous results. The signals at the higher vibrational lines are extremely noisy, and there is uncertainty in assigning band positions due to the non-negligible rotational band contours. Additionally, some of the bands fall at positions grossly out of line with other progression members (e.g., the 3, 1 band), as if there is some perturbation. Therefore, certain bands are omitted from the fitting procedure to determine the constants (e.g., 3, 1, 4, 0 and 5, 0 in the E *-- X system, and 3, 0 and 4, 0 in the F *--X system). With the limited number of acceptable line positions measured, extrapolation to the dissociation limits in these excited states is therefore unwarranted. The ground state vibrational constants can be obtained from the several hotband positions measured. In the lower-energy region of the E ~ X system, six hotbands other than 3, 1 are measured, with transitions originating from the ground state levels v" = 0 - 3 . The measured v" = 3 level places a rigid lower limit on the binding of D O > 488.5 c m (1.4 kcal/mol). Using the vibrational constants determined and the relationships D e = WeZ/(4Wex e) and D o = D e - (o~/2) + (O~eXe/4), we determine a result equivalent to a Morse extrapolation for the ground state dissociation limit. This result relies on the hotband positions, which have been measured more

accurately in the highly averaged region of the spectrum, and using bands which do not appear to be perturbed (Table 3). A value of D~ = 1330 cm-1 is obtained, which indicates that we have extrapolated 840 c m - I beyond the last observed band. This value is of course still subject to uncertainty because of rotational contour linewidths and the assumption of a Morse potential, but past experience indicates that these effects will not add a large error to this value. The dissociation energy and vibrational frequency determined here for the ground electronic state of LiMg are in excellent agreement with theory. Jones has reported D O = 0.17 eV and we = 180 c m - i [4]. Fantucci et al. have reported D O = 0.12 eV [7]. Bauschlicher et al. have reported D O = 0.188 eV and o)e = 183 cm-~ [8]. Boldyrev et al. reported D O = 0.18 eV and (oe = 121 cm 1 [9]. Our values of D'~= 1330 cm -1 (0.165 eV; 3.80 k c a l / m o l ) and We = 190 cm ~ fall very close to all of these values. On the other hand, the previously reported thermochemical value [10] for the dissociation energy of D O = 15.2 k c a l / m o l (0.659 eV) must be regarded as unreasonably high. Theory should be able to do a good job on a molecule this small, and it is inconceivable that our ground state vibronic level extrapolation could be off by this much. Table 4 shows a comparison of the properties measured for LiMg to the known parameters for Li2 and Mg 2 [3]. As noted earlier, the ground 2E+ state

Table 4 Comparison of the ground state spectroscopic constants for Li 2, Mg 2 and LiMg

oJ~' ( c m - l ) r~' (A) D~ (eV)

7Li2

7Li24Mg

24Mg 2

351.4 2.67 1.04

190.0 0.165

51.1 3.89 0.05

K.R. Berry. M.A. Duncan/Chemical Physics Letters 279 (1997) 44-49

has a bond order of 1/2, and the bond energy is therefore expected to weak. However, the comparison shows that this bond energy is much less than half of the single-bond value in Li 2- Apparently, the valence orbitals of Li and Mg are very ineffective in bonding. We have noted similar weak bonding between Li and A1 in the diatomic LiA1, where there is a full single bond but the dissociation energy is also relatively low (0.75 eV) [14]. In the only other spectroscopic investigation of LiMg, Pichler et al. have measured green chemiluminescence attributed to C and D state emission (bound-free) from a heat pipe containing lithium and magnesium [6]. After excitation of Li 2 states, collisional excitation transfer was postulated to produce LiMg in the C a n d / o r D states, which emitted to the ground state. However, the excitation wavelengths used in their study (a variety of Ar 2+ laser lines in the 333-360 nm region) are not high enough in energy to reach the origins of the states measured here. The interpretation of these authors, i.e., that the C and D states are emitting, is therefore reasonable in light of our results. It would therefore be interesting to investigate the LiMg system in the region of these lower C and D states with our R2PI techniques, but these experiments would likely require two-color ionization. It is disappointing that the spectroscopy of LiMg in this energy region is limited by signal-to-noise and by the contamination from the underlying continuum absorption. However, scans to higher energy may make it possible to locate molecular states correlating to the Li ( 2 S ) + Mg(~P) asymptote. The corresponding magnesium resonance line has extremely large absorption strength, and the related molecular band systems should therefore gain significant intensity. Future studies will investigate this

49

wavelength region with the hope of obtaining higher-quality spectra and perhaps rotationally resolved spectra.

Acknowledgements Research support from the U.S. Air Force Office of Scientific Research (grant Nos. F49620-94-1-0063 and F49620-97-1-0042) is gratefully acknowledged.

References [1] M.D. Morse, Chem. Rev. 86 (1986) 1049. [2] M.D Morse, in: M.A. Duncan (Ed.), Advances in Metal and Semiconductor Clusters, vol. 1, JAI Press, Greenwich, CT, 1993, p.83. [3] K.P. Huber, G. Herzberg, Molecular Spectra and Molecular Structure, IV. Constants of Diatomic Molecules, Van Nostrand Reinhold, New York, 1979. [4] R.O. Jones, J. Chem. Phys. 72 (1980) 3197. [5] B.K. Rao, P. Jena, Phys. Rev. B 37 (1988) 2867. [6] G. Pichler, A.M. Lyyra, P.D. Kleiber, W.C. Stwalley, R. Hammer, K.M. Sando, H.H. Michels, Chem. Phys. Lett. 156 (1989) 467. [7] P. Fantucci, V. Bonacic-Koutecky, W, Pewestorf, J. Koutecky, J. Chem. Phys. 91 (1989)4229. [8] C.W. Bauschlicher, S.R. Langhoff, H. Partridge, J. Chem. Phys. 96 (1992) 1240. [9] A.I. Boldyrev, J. Simons, P.V.R, Schleyer, J. Chem. Phys. 99 (1993) 8793. [10] C.H. Wu, H.R. Ihle, Proc. 8th Int. Mass Spectrom. Conf., Oslo 8A (1980) 374. [11] L.R. Brock, A.M. Knight, J.E. Reddic, J.S. Pilgrim, M.A. Duncan, J. Chem. Phys. 106 (1997) 6268. [12] K. LaiHing, R.G. Wheeler, W.L. Wilson, M.A. Duncan, J. Chem. Phys. 87 (1987) 401. [13] C.E. Moore, Atomic Energy Levels, National Standard Reference Data Series 35, National Bureau of Standards, Washington, DC, 1971. [14] L.R. Brock, J.S. Pilgrim, M.A, Duncan, Chem. Phys. Lett. 230 (1994) 93.