darkhistory.physics.He_exc_lambda

darkhistory.physics.He_exc_lambda = {'21p': 5.843343570072678e-06, '21s': 6.014044949187253e-06, '23p': 5.914115218792692e-06, '23s': 6.255631064949655e-06}

HeI n=1 to n=2 excitation wavelength in cm.