JEE Advanced 2021 Full Test 4 Paper 2

A photoelectric plate is initially exposed to a spectrum of hydrogen gas excited to second energy level. Later when the same photoelectric plate is exposed to radiation from some unknown hydrogen like gas, excited to second energy level, it is found that the de-Broglie wavelength of the photoelectrons, now ejected has decreased √6.1 times. For this new gas difference of energies of first Lyman series and Balmer series, limit is found to be two times, the ionisation potential of the hydrogen atom. Find the value of Z for unknown gas.