A NEW ANALYSIS OF A VERY OLD SPECTRUM: THE HIGHLY PERTURBED A 2  i – X 2  i BAND SYSTEM OF THE CHLORINE CATION (Cl 2 ) Mohammed A. Gharaibeh and Dennis J. Clouthier + DEPARTMENT OF CHEMISTRY

Background Plays an important role in etching processes using chlorine containing plasmas The electronic spectrum ( 2  u - 2  g ) was first reported in 1928 The first excited state is vibrationally perturbed although the rotational band structure is regular Upper state vibrational numbering, bond length and perturbation matrix elements unknown Chlorine cation (Cl 2 ) +

πg*πg* πuπu Cl 2 MO diagram + σ*σ* X 2  g A 2  u SHOMO HOMO LUMO

Production of jet-cooled chlorine cation Pulsed valve Ring electrodes 40 psi of 10% Cl 2 in Ar HV + Cl 2 Vacuum (~10 -6 torr)

Low resolution LIF spectra of Cl Cl Cl 37 Cl + 37 Cl 2 + weak bands Good signal to noise ratio Cold spectrum No transitions from X 2 Π 1/2

A 2  3/2 – X 2  3/2 vs A 2  1/2 – X 2  1/2 2Пg2Пg 2Пu2Пu Ω =1/2 Ω =3/ cm -1

A B C D The red end of the Cl 2 spectrum What do the emission spectra tell us? Is this the 0-0 band? cm -1 What do the isotope shifts ( 35 Cl + – 35 Cl 37 Cl + ) tell us? 2

Nodes in the SVL emission of 35 Cl 2 + A B C D node

Observed isotope shifts (cm -1 ) Where is the 0-0 band? Calculated isotope shifts (cm -1 ) v′=0 v′=1 v′=2 v′=3 v′= ω′ = cm -1 ω x′ = 4.19 cm -1 e e e e e e ω″ = cm -1 ω x″ = 2.93 cm -1 A B C D v′=1 v′=2 v′=3 v′=4

v′=0 v′=1 v′=2 v′=3 ω′ = cm -1 ω x′ = 4.19 cm -1 e e e Obs-Calc Isotope shifts (cm -1 ) B′ = Nature of the perturbations in 35 Cl 2 + Extra Band

Perturbation matrix element for Cl W : perturbation matrix element W W Extra Band

Nature of the perturbation in 35 Cl 2 + W : perturbation matrix element = 119 cm -1 Calculated isotope shift : Observed isotope shift : Calculated B′ : Observed B′ :

R. P. Tuckett and S. D. Peyerimhoff, Chem. Phys. 83 (1-2), 203 (1984) Bond length (A) o Potential curves of Cl 2 2g2g 2  u (  =3/2) 2  u (  =3/2) E (Hartree) (  g ) 2 (  u ) 4 (  g ) 3 (  g ) 2 (  u ) 4 (  g ) 2 (  ) 1 ** (  g ) 2 (  u ) 3 (  g ) 4 * * + Homogeneous perturbation,  =0 Perturbation matrix element is ~ 120 cm -1

Emission spectra of 35 Cl Parameter 35 Cl 2 35 Cl 37 Cl ωeωe (12)637.54(23) ω e x e (69)2.938(14) N1511 Ground state vibrational parameters + + ″ ″

35 Cl + band head 35 Cl 37 Cl + band head 2 Resolving the overlapped isotopologues

Synchronous-scan LIF technique Pump Laser Tunable Dye laser PMT Laser: Offset: 450 cm -1 Monochromator PMT

Total LIF Sync-scan 35 Cl 37 Cl + Sync-scan 35 Cl 2 + Synchronous-scan LIF of Cl band

High resolution LIF spectra of Cl 2 T=30 K FWHM=0.03 cm Cl 2 (1-0 band) + +

Rotational constants and molecular geometry Parameter (cm -1 ) 1- 0 band2- 0 band 35 Cl + B′′ (3) (5) Origin (9) (1) B′ (3) (4) 35 Cl 37 Cl + B′′ (3) (5) Origin (9) (1) B′ (3) (4) * F. P. Huberman, J. Mol. Spectrosc. 20 (1), 29 (1966). r e =1.8918(4) Å *r e =2.2287(7)Å Ground state 1 st Excited state ″ ′ 2

1- LIF spectra of jet-cooled Cl 2 have been extended 3700 cm -1 to the red of previously reported observations. 2- The first three bands are unperturbed, including the 0-0 band at cm -1, and the vibrational numbering has been established. 3- The equilibrium bond length for the cation in its first excited state was determined to be (7) Å. 4- The levels of the 2  3/2 component of the 2  u excited state are perturbed by a near-by 2  u state with Δ  =0 and matrix elements of about 120 cm -1. Summary +

Isotope shifts