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

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+.

Ionization makes the Th-O bond weaker but stiffer IE(Th)=6.3067 eV IE(ThO)=6.6027 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 895.77 ThO+ 954.97

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

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

Time-of-flight mass spectrum showing the products from pulsed laser ablation of Hf Isotopes Hf % 174 0.2 176 5.3 177 18.6 178 27.3 179 13.6 180 35.1 (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.

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.

Comparison of LIF and REMPI spectra HfO Comparison of LIF and REMPI spectra G1S (v=0) – X 1S (v=0) Hf % 174 0.2 176 5.3 177 18.6 178 27.3 179 13.6 180 35.1

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 27353 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+.

Vibrationally resolved photoelectron spectrum for HfO Tv+ = Te + we(v+1/2) – wexe(v+1/2)2 we = 1017.7 cm-1 wexe = 3.2 cm-1 No evidence for excited electronic states Excitation via F1S (v=2)  X1S (v=0)

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

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

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

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 0.386537(7) B0+ .40959 .39473 .40807 .40422 .39722 .40061b .39432c .39601 0.403(5) DG1/2 1036.69 989.67 1043.11 1024.93 988.62 989.56b 976.76c 969.75 974.09 DG1/2+ 1112.78 1004.54 1099.09 1077.03 1027.35 1030.19b 1015.81c 1019.79 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 7.91687(10) CASSCFd MRCISDd MRCISD(Q) CCSDT 6.480c,e 7.357 c,e 7.631 c,e 7.755 c,e

Comparison of measured properties for HfO and ThO HfO/HfO+ ThO/ThO+ IE(eV) 7.91687 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.

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+.