Consider a helium (He) atom that absorbs a photon of wavelength...

$P = \dfrac{h}{\lambda}$

$mv = \dfrac{h}{\lambda}$

$v = \dfrac{h}{m \lambda}$

$v = \dfrac{6 .6 \times {10}^{- 34} \times 6 \times {10}^{23}}{4 \times {10}^{- 3} \times 330 \times {10}^{- 9}}$

$v = 3 \times {10}^{- 1}m / s$

$v = 30cm / s$

$∆ v = {v}_{f} - {v}_{i} = v - 0 = v = 30cm / s$