The ionization potential of Ag2

Volume 185,number 3,4

CHEMICALPHYSICSLETTERS

18 October 199 I

The ionization potential of Ag, V. Beutel, G.L. Bhale ‘, M. Kuhn and W. Demtrtider Fachbereich Physik, Universitiit Ka~serslautern, W-6 750 Karsersiautern. Germany

Received 3 1July 199I

Silver dimers Ag, formed in a seeded supersonic argon beam are excited into autoionizing Rydberg levels by resonant two-step transitions with two pulsed dye lasers. The three isotopes of A& are separated by a time of flight mass spectrometer. From the convergence limits of different Rydberg series converging towards different vibratIona

levels U+ in the X ‘Zz ground state of

Ag; the vibrational constants of the ion ground stateand the adiabatic iomzation potential IP( Ag: )=61747&4 cm-’ could be accurately determined.

1. Introduction The ionization energies of molecules and clusters have been a matter of intense research [ 1,2] and often controversial results have been published, which were mainly caused by the differences between adiabatic and vertical ionization energies [ 3 1, Several experimental techniques have been used so far for the determination of ionization potentials. One method is based on the direct one-photon ionization starting from thermally populated levels (u”, J” ) in the ground state of the neutral molecule. Since the thermal population distribution N(v”, J”) becomes very broad in vapor cells or furnaces at high temperatures, the measured ion yield N + (A), i.e. the number of ions produced as a function of the ionizing laser wavelength 2, follows a rather flat curve, determined by the convolution of population densities N( u”, J” ) and ionization probability. In such cases the onset of this curve, which yields the ionization potential IP, cannot be determined very accurately. In cold supersonic beams the population distribution N( u”, J” ) is considerably narrowed, because of the low rotational and vibrational temperatures achieved in seeded beams [4]. This enhances the

slope of the ion yield N + (1) and allows a more accurate determination of its onset. However, the main problem is still the uncertainty in the correct assignment of the vibrational level v+ reached in the ion ground state. If the potential curve E,(R) of the ion ground state is shifted against that of the neutral ground state, vertical transitions, favored by the Franck-Condon principle, generally end in excited levels v+ > 0 of the ion ground electronic state and the measured ionization threshold is too high. A more selective ionization can be achieved by resonant two-photon two-colour ionization with tunable lasers. The first laser LI is tuned to a selected transition ( v’, J’) t (v”, J” ) from the neutral ground state into an intermediate electronic state and a second tunable laser L2 ionizes the excited molecules in level (v’, J’). This method has the definitive advantage that different levels (v’, S ) of the intermediate state can be selectively populated, thus probing different vertical transitions into different levels V’ in the ion ground state. A comparison of the measured onsets of the ionization yield curve N+(A) generally allows the correct assignment of v+ and therefore a more reliable determination of the adiabatic IP, which is defined as the term difference lP=T(v’=O,J+=0)-T(v”=O,J”=O).

’ Permanent address: Spectroscopy Division, Bhabha Atomic Research Center, Trombay, Bombay 400085, India. 0009-2614/91/S

There is, however, another problem which affects the measured value of IP: since the ions or photoelec-

03.50 0 1991 Elsevier Science Publishers B.V. All rights reserved.

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trons are extracted from the ionization volume to the detector by an electric field E, the appearance potential AP of the ions, AP=IP-

(e'E/m,,)'/'

,

(1)

is lowered against the true field-free IP by an amount ( e3E/nq,) ‘I2 due to field ionization [ 51. This makes it necessary to measure AP at different electric fields and to extrapolate the results to E-+0. These problems cunn~t be avoided by using pulsed electric fields which are switched on only after the excitation has ended. Because of the long lifetimes of high Rydberg levels they still can be field-ionized even at delay times of several microseconds. The most accurate method for the determination of the ionization potential IP is based on measurements of different series of autoionizing Rydberg levels. The term values T(n, v*, J*) of these levels with principal quantum number n, vibrational and rotational quantum numbers v*, J* converge for n+co towards the level (v+=u*, J’=J*-I) of the ion ground state, where I is the angular momentum of the outgoing electron [ 6 1. The present paper reports on the accurate deter-

mination of the IP of the Ag, dimer, using this method of converging Rydberg series.

2. Experimental The experimental setup is shown in fig. 1. The Ag, molecules are formed during the adiabatic expansion of silver vapor seeded in argon through a 60 pm nozzle from a high-temperature oven (PA,%4 bar, Tz2000 K) into the vacuum chamber. The radiation shielded oven is made of TZM alloy (tantalum, zyrconium and molybdenum) which can be heated up to temperatures of i”< 2200 K by radiation heating from tungsten bows. The molecular beam is collimated by a skimmer and is then crossed perpendicularly by the two superimposed beams of two pulsed dye lasers. The first frequency doubled dye laser (coumarin 153) populates levels ( ~1’ , J’), in the B ‘C, state of Ag2 and the second dye laser L2 (QUI) induces transitions from these levels into autoionizing Rydberg levels. Since silver has two natural isotopes, “‘Ag and lo9Ag,of about equal abundance, the excitation spectra of the resulting 3 molecular isotopes partly overlap [ 61. In order to achieve isotope-selective exci-

Ag-oven

I

P

Fig 1. Experimentalarrangement. 314

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CHEMICALPHYSICSLETTERS

Volume185,number 3,4

tation of defined intermediate levels, isotope separation by a mass spectrometer is used. The Ag: ions produced within the crossing volume of lasers and molecular beams are extracted by an electric field and drift through a 75 cm long time of flight mass spectrometer of the McLaren type [ 7 1, which has two separated acceleration parts in order to compensate for the extension of the ionization volume. The spread of initial ion velocities is negligible because of the molecular beam collimation. Fig. 2 shows the mass spectrum of the three molecular isotopes ‘07.4g’07Ag,“‘Ag’O’Ag and “‘Ag’O’Ag. This isotope selection allows one to set the first laser onto the band head of a wanted vibrational band of a specific isotope and to populate only a few overlapping rotational levels with quantum numbers J in a selected vibrational level of the B ‘C, state. In our case rotational levels in the range 7
v*

principal

is recorded as a function of Al. Such a spectrum of partly overlapping Rydberg series from the intermediate level B ‘C,( v’= 4) to Rydberg levels R (n, v*) with different vibrational quantum numbers v* is shown in fig. 3.

Ag 107 - Ag 109

A

31.6

31.6

32

32.2

32.4

32.6

32.8

Limeof flight In bs Fig. 2. Mass spectrum ofthe three molecular isotopes ‘07Ag’07Ag, ““Aglo9Agand ‘0sAg’09Ag.

quantum

number

I

61800

62000 Wavenumber

62200

62400

62600

in cm-’

Fig, 3. Rydberg spectrum of transitions starting from u’= 4 in the B ‘I, state. This spectrum consists of seven Rydberg series converging towards v+ = 3,4, .. .. 9.

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Volume 185, number 3,4

3. Results and discussion The term values T( n, u*) of the Rydberg levels are given by the Rydberg formula

Table 1 compiles all relevant quantities obtained from fits to eqs. (2) and (4). The mean values obtained for IP( v’ =O) and the quantum defects for the isotope ‘07Ag’09Agare

T(n, V*)=IP(U*)-R/[n-6(v*)]*)

IP(v’=O)

(2)

where R=l09737.0366 cm-’ is the Rydberg constant for the “‘Ag”‘Ag isotope and 6(v*) is the quantum defect [S] which is caused by different contributions, such as the quadrupole moment of the molecular ion core, the polarizability of the curve, exchange effects between the Rydberg electron and core electrons and by perturbations, due to I-mixing or to doubly excited configurations [ 93. The quantum defects are treated here as pure fit parameters which were obtained together with the values of IP( u*) when fitting eq. (2) to the experimental term values 7’(n, v*). The convergence limits IP( v*= v+) can be written as IP(v+)=IP(v+=O)tG(t~+)-G(v+=O).

(3)

A Birge-Sponer plot of all measured vibrational term differences, AG(v+)=G(v++l)-G(v+) =0,-2w,x,(v++1))

(4)

yields the vibrational constants w, and w&~ of the Ag: ground state. With these constants the best fit values of G( U+ ) were obtained which in turn yield the adiabatic ionization potential IP(u+=o,J+d17) =IP(v+)-[G(v+)-G(v+=O)]

.

(5)

=61747.1&0.3cm-

k4 cm-’ (3olimit+other

AIP(v+=O,J+=O)<(B;-B;)J(Jtl), is therefore less than 2.0 cm -‘. From the Birge-Sponer plot of AG(v’) (fig. 4) the following vibrational constants of the Ag: (%9+) are obtained:

and the adiabatic ionization potential

V+

J(u+)

IP(v+) (cm-‘)

G(v+)-G(0) (cm-‘)

IP(v+)-G(v+)tG(O) (cm-‘)

4 5 6 I 8 9 10 I1

0.79*0.02 0.78f0.02 0.78+0.01 0.76f0.01 0.76+0.01 0.75+o.oL 0.75~0.01 0.76iO.01

62280.8 k 1.0 62410.8k 1.0 62540.9f0.9 62670.0 k 0.7 62797.5 i-O.7

533.2 664.0 793.8 922.6 1050.4 1177.2 1303.0 1427.8

61747.6 61746.8 61747.1 61747.4 61747.1 61746.9 61746.8 61746.9

62924.1 kO.6 63049.8 f0.7 63 174.7 k 0.9

errors) ,

The statistical standard deviation of the fit ( I (T) for IP(v+) is 0.3 cm-‘. Other error source also contribute to the uncertainty. These are: (1) The uncertainty of the wavelength measurements of both lasers Ll and L2, which was performed with a Fabry-Perot wavemeter, which, hawever, was not accurately calibrated in the ultraviolet region between 380-395 nm where L2 was operated. The uncertainty here is about 2.0 cm-‘. (2) The contribution of the rotational term values with J< 17 to the measured term values. From rotationally resolved spectra the rotational constant of the X ‘C, ground state of Ag, could be determined as Bgz0.0512 cm-’ [lo]; that ofthe Ag:(*E:) ion ground state is estimated to be B:~0.045cm-‘. The rotational contribution to the uncertainty of the adiabatic ionization potential.

Table 1 Quantum defects &vi), ionization potentials IP(v+). vibrational term values G( v+)-G(0) IP(0) = IP( v’) - G( v’) -G(O) as determined from tbe different Rydberg series

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Volume 185, number 3,4

R

delta C(vt)

E = 90 V/cm

135 7

131 ;

-----f

130 f

1E = 120 V/cm 129 ! 128 : 127 f 126 I 125 124 123

E = 170 V/cm

122 I 1

2

3

4

5

7

6

8

9

10

11 12 13

vt Fig. 4. Birge-Spotter plot ofvibrational spacings in the ‘Z: ground

state of Ag2+. co,= 135.8kO.3 cm-‘,

, w,.x, =0.50?0.02

cm-’ .

61650

61700

61750

61800

61850

61900

61950

wavenumber in l/cm

From the adiabatic ionization potential IP (Ag, ) and the atomic ionisation energy IP (Ag) the difference between the dissociation energies,

Fig. 5. Dependence of the appearance potential on the electric field strength of the extraction field. The spectra correspond to transitions starting from the intermediate level u’=2.

D,(Ag,(‘Z,+))-D,(Ag:(2~,+)) =IP(Ag,)-IP(Ag), can be obtained. With the values [ 111 IP(Ag*)=61747cm-‘,

IP(Ag)=61106cm-

this difference becomes Do(Ag2(‘C:))-Do(Ag:(2Z,C))=641

cm-‘.

The ion ground stale is therefore slightly less bound than the neutral ground state. With the absolute value &(Ag,(‘C,+))=13307&250cm-’ taken from ref. [ 121, one obtains the absolute value Do(Ag:(2C,+))=12666*250cm-‘, where the error limits are determined by the uncertainty of Do(Ag,).

The last point, which has to be discussed, is the influence of the electric field on the measured term value. In fig. 5 three identical sections of the Rydberg spectra, excited by laser L2 from the intermediate level v’=2 are shown at three different field strengths of the extraction field. The increasing field ionization with increasing field can be clearly recognized. The appearance potentials are measured from the onset points of the ion currents. They agree very well with the values obtained from eq. ( 1) with the ionization potential IP taken from the Rydberg series which are indicated by dashed vertical lines. This gives, in addition, an extra confirmation of the correct v+ assignment of the Rydberg series. However, the determination of AP is by far not as accurate as that of IP. This is partly due to the fact that the direct ionization, which gives a continuous background, is overlapped by the much stronger au317

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toionization lines, and the onset of the ion current N(I,) is much steeper, if an autoionization line just coincides with this onset at the AP, than if AP lies between two lines (as, for example, the second curve in fig. 5). With very few exceptions no measurable influence of the electric field on the position of the autoionization lines could be detected. This can be explained as follows: the electric Stark shift of the Rydberg levels is A W= al?, where the electric polarizability a(n) scales with n’. For levels with n 6 30 the Stark shift is apparently less than 0.1 cm- ‘. In a few cases there may be a close coincidence between levels (n ,, VT) and ( n2, vf ) with U; > v;, n, < n2 which then can be mixed by the electric field [ 131. This, however, does not influence the convergence limits of our measured Rydberg series and thus does not affect the accuracy of the ionization potential IP( v+=O).

Acknowledgement This work was supported by the Deutsche Forschungsgemeinschaft within the Sonderforschungsbereich 91. We thank H.J. Miischenborn and W. Theiss for advice and help within the lasers.

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References [ I ] K.I. Peterson. P.D. Dao, R.W. Farley and A.W. Castleman Jr., J. Chem. Phys. (1984) 1780. [2] M.M. Kappes, M. Schar, U. Rothlisberbger,Ch. Kerzetzian and E. Schumacher,Chem. Phys. Letters I43 ( 1988) 25 1. [ 31 M. Sander, L.A. Chewter, K. Miiller-Dethlefsand W. Schlag, Phys. Rev. A 36 (1987) 4543. [ 41 D.H.Levy,L. Wharton and R.E. Senally,Laserspectroscopy in supersonicjets, in: Chemicaland biochemicalapplications oflasers, Vol.2, ed. C.B. Moore (AcademicPress, NewYork, 1977); G. Stoles, ed., Molecular beams (Oxford Univ. Press, Oxford, 1990). [ 51M. Broyer, J. Chevaleyre, G. Delacretaz, S. Martin and L. Wiiste, Chem. Phys. Letters 99 ( 1983) 266. [6] R.F. Stebbings and F.B. Dunning, Rydberg states of atoms and molecules(Cambridge Univ. Press, Cambridge, 1983). [7] W.C. Wiley and I.H. McLaren, Rev. Sci. Instr. 26 (1955) 1150; M. Kato, A. Mogami and M. Naito, Rev. Sci. Instr. 59 (1988) 1947. [8] M.J. Seaton, Proc. Phys. Sot. 88 (1966) 801. [9] Ch.H. Green and Ch. Jungen, Advan. At. Mol. Phys. 21 (1985) 51. [IO] CM. Brown and M.L. Gutter, J. Mol. Spectry. 69 (1978) 25; D.S. Pesuc and B.R. Vujisic, J. Mol. Spectry. 146 ( 1991) 5 16,and references therein. [ 111C.E. Moore, Atomic energy levels. Nat. Stand. Ref. Data Ser. 35 (Nat. Bur. Stds., Washington, 1971). [ 121M.D. Morse, Chem. Rev. 86 (1986) 1049. [ 131G.R. Janik, D.C. Mullins, CR. Mahon and T.F. Gallagher, Phys. Rev. A 35 ( 1987) 2345.