Ionization Potentials for Protactinium

[atomicpedals]

I'm trying to compile the first five IP's for Protactinium.

I was able to find 1-3 on these two sites

http://www.astro.lsa.umich.edu/~cowley/ionen.htm
http://www.gordonengland.co.uk/elements/pa.htm

But am coming up with nothing for the 4th or 5th IP. Does anyone have any possible sources for 4 and 5? I checked CRC and they only have the 1st IP, ditto for NIST.

 

[atomicpedals]

Found references for them in:

G.V. Ionova et al. J. Phys. Chem. 96 (1992) 11096-11101.