Since the outermost electron of alkali metals always feels the same effective nuclear charge (+1), the only factor which affects the first ionisation energy is the distance from the outermost electron to the nucleus.
sˈɪns ðɪ ˈaʊɾɚmˌoʊst ᵻlˈɛktɹɑːn ʌv ˈælkəlˌaɪ mˈɛɾəlz ˈɔːlweɪz fˈiːlz ðə sˈeɪm ɪfˈɛktɪv nˈuːklɪɹ tʃˈɑːɹdʒ plˈʌs wˈʌn, ðɪ ˈoʊnli fˈæktɚ wˌɪtʃ ɐfˈɛkts ðə fˈɜːst ˌaɪənəzˈeɪʃən ˈɛnɚdʒi ɪz ðə dˈɪstəns fɹʌmðɪ ˈaʊɾɚmˌoʊst ᵻlˈɛktɹɑːn tə ðə nˈuːklɪəs.