What is the threshold frequency ν0 of cesium?

 I think this is what you're after:

 Cs(g) → Cs^+ + e⁻ ΔHIP = 375.7 kJ mol^-1 [1] 

Convert to J and divide by the Avogadro Const to give E in J per photon 

E = 375700/6.022×10^23 = 6.239×10^-19 J 

Plank relationship E = h×ν E in J ν = frequency (Hz s-1) 

Planck constant h = 6.626×10^-34 J s 

6.239×10^-19 = (6.626×10^-34)ν 

ν = 9.42×10^14 s^-1 (Hz) 

IP are usually given in ev Cs 3.894 eV 

E = 3.894×1.60×10^-19 = 6.230×10^-19 J per photon 

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