Actually those elements are added as oxides, i.e. 2% Thoriated = 2% ThO2, but also Ceriated = 2% CeO2 or Zirconiated = 8% (ZrO2); etc.

The addition of those elements enhance the "electron emission capacity", improving the stability of the arc and also can help in arc initiation (you can see the difference in went try to weld Aluminum or Titanium using pulsed current). As the arc is more stable, are less change of located electrode overheating and melting, helping to maintain the shape of the tungsten electrode tip (generally sphere).

As we know, the Ceria (CeO2) can change from valence +3 to +4; the Zirconia (ZrO2) have a high ion-exchange capacity... those properties can influence on the electron shell of the tungsten, decreasing his Ionization Energy (as you call ionization potential), increasing the electron emission capacity.

Some of these tungsten electrodes are wonderful on AC current but at the same time the worst election on DC current (or DCEN, etc), so always check the recommendation or you can melt your electrode very easy.

I highly recommend the AWS standards,

PD: take care when grinding these electrodes, they contain low-level radiative materials

Thoriated tungsten produces a stable arc and improves the current carrying capacity. Wandering of arc is reduced as well.

why thoriated tungsten electrode is detected as bismuth tungsten electrode? I am using XRF method to characterize it.

My guess is that the weld penetration might be lower for the thoriated electrode as compared to the pure W one for the same current.