1. Ionization Energy Measurements and Spectroscopy of HfO and HfO+
Jeremy M. Merritt,
Vladimir E. Bondybey,
and Michael C. Heaven
DoE

2. Motivation
Previous studies of ThO ionization revealed unexpected results. Ionization of the molecule weakened the bond (D0-D0+=0.3 eV), but the vibrational frequency for the ThO+ ion was higher (we+=955 vs. we=896 cm-1) and the bond length was shorter (Re+=1.807 vs. Re=1.840 Å). Franck-Condon principle violations were observed in the photoelectron spectrum.
Is this unusual behavior of ThO simply a consequence of the electronic configurations involved or are relativistic effects playing a significant role?
A comparison of Hf2+(6s2)O2- with Th2+(7s2)O2- can be used to probe this question. Ionization energy (IE) measurements by Rauh & Ackerman indicate that D0(HfO)>D0(HfO+). In the present work we obtained an accurate IE and the first spectroscopic constants for HfO+.

3. Ionization makes the Th-O bond weaker but stiffer
IE(Th)= eV
IE(ThO)= eV
D0+
IE(Th)
Hence, the ThO+
bond is weaker
Th + O
D0-D0+=0.296 eV
but its vibrational
frequency is higher D0
IE(ThO)
e /cm-1
ThO
ThO

4. Experimental Techniques
REMPI
& ZEKE
LIF
Pulsed laser vaporization of metal samples.
Laser induced fluorescence spectroscopy of neutral species.
Two color resonance enhanced multi-photon ionization spectroscopy with mass selection.
Pulsed-field ionization zero kinetic energy photoelectron spectroscopy

5. Multi-Photon Ionization Processes
HfO+ + e-
Pulsed
electric
field
hv2
hv2
HfO*
hv1
hv1
HfO
REMPI
ZEKE
PIE
MATI

6. Time-of-flight mass spectrum showing the
products from pulsed laser ablation of Hf
Isotopes
Hf %
(Hf)n+ clusters up to n=6 are observed, as well as signals due to (Hf)nO+, upon non-resonant ionization with 193 nm light. The inset shows a higher resolution mass spectrum recorded with a longer flight tube to aid in separating the 6 naturally occurring isotopes of Hf. Resonant excitation of the G-X band of HfO has been used in this case. The peaks observed when the gas pulse is turned off are due to background impurity molecules.

7. Laser induced fluorescence spectrum
for jet-cooled HfO
Survey scan in the region of the E-X and F-X bands systems of HfO. The peak marked with a † is assigned to the D X 4-0 transition. Sequence band transitions marked with a ‡ have been tentatively assigned as originating from a metastable triplet state. Peaks marked with an asterisk are due to atomic Hf.

8. Comparison of LIF and REMPI spectra
HfO
Comparison of LIF and REMPI spectra
G1S (v=0) – X 1S (v=0)
Hf %

9. Two-color photoionization of HfO
IE=63760 cm-1
Photoionization efficiency (PIE) spectra for HfO recorded with the first laser tuned to the P(3) line of the
F(0+) (v’=0) X 1S(v”=0) band at cm-1. The insert shows part of the spectrum recorded under higher resolution illustrating the sharp resonance structure above the ionization threshold. The different traces in the inset correspond to gating on the different isotopomers of HfO+.

10. Vibrationally resolved photoelectron
spectrum for HfO
Tv+ = Te + we(v+1/2) – wexe(v+1/2)2
we = cm-1
wexe = 3.2 cm-1
No evidence for
excited electronic
states
Excitation via F1S (v=2)  X1S (v=0)

11. Rotationally resolved photoelectron spectrum for HfO
Rotational structure confirms that the electronic ground state for HfO+ is X2S+.
Rotational constant
for the ion is
B0+ = 0.403(5) cm-1
1.687(3) Å

12. IE for HfO is greater than the literature value
Electron impact
PFI-ZEKE
6.1(1) U
6.194
5.6(1) UO
6.031 Th
6.307
ThO
6.604
6.65(10) Hf
6.825
7.55(10)
HfO
7.917

13. Electronic structure calculations
for HfO and HfO+
Hf = ECP60MWB
(8s7p6d2f1g)/[6s5p3d2f1g]
Hf: 5s2 5p6 5d2 6s2
O = aug-cc-pVTZ
Molpro

14. Computed properties of HfO and HfO+
All constants are given in cm-1 units except for the
ionization energy which is in eV. HF
MP2
CISD
CCSD
CCSD(T)
B3LYP
Expa B0
.39171
.38566
.39350
.39017
.38475
.38775b
.38180c
.38242
(7)
B0+
.40959
.39473
.40807
.40422
.39722
.40061b
.39432c
.39601
0.403(5)
DG1/2
989.67
988.62
989.56b
976.76c
969.75
974.09
DG1/2+ b c
1013(1) IE
6.7810
7.5466
7.3037
7.361e
7.374c,e
7.6687
7.7365
7.7371b
7.7485c
7.753c,e
7.7286
(10)
CASSCFd
MRCISDd
MRCISD(Q)
CCSDT
6.480c,e
7.357 c,e
7.631 c,e
7.755 c,e

15. Comparison of measured properties for
HfO and ThO
HfO/HfO+
ThO/ThO+
IE(eV)
6.6026
D0-D0+ (eV)
1.092
0.2957
we+(cm-1)
1017.7
954.97 we
974.09
859.77
Be+
0.403
0.3451 Be
0.3865
0.3326
Both HfO and ThO exhibit MO+ bonds that are weaker and stiffer
than those of the neutral molecules. This behavior is associated with the ns2 (n-1)d2 configuration of the metal atom.

16. Conclusions
The HfO IE measured by PFI-ZEKE is 0.37 eV higher than previous estimates from electron impact measurements.
The Hf-O bond is weakened by ionization. However, the bond length contracts and the vibrational frequency increases. This is the same as the anomalous behavior observed for ThO.
Franck-Condon violations observed in the photoelectron spectrum of ThO were not present in the spectra for HfO. This difference is attributed to a mixing of ionic and neutral states of ThO which is not possible for the excited levels of HfO+.