I am reading the article named Manipulating Atoms with Photons by Claude Cohen-Tannoudji and Jean Dalibard. On page no. 16, the following is said.
Consider the atom in its ground state $g$ and its center of mass initially at rest; suppose that a photon with wave vector $\textbf{k}$ is sent on this atom. If the atom absorbs the photon, it jumps to the excited state and it recoils with the momentum $\hbar\textbf{k}$.
My confusion
I am confused about the direction of the recoil momentum.
To be specific, let's say the photon's wave vector is $\textbf{k} = k\hat{x}$, where $\hat{x}$ is the unit vector. If the atom absorbs the corresponding momentum $\textbf{p}_\text{photon} \equiv \hbar\textbf{k} = \hbar k \hat{x}$, then I think the recoil momentum is $\textbf{p}_\text{recoil} = -\hbar\textbf{k} = -\hbar k \hat{x}$, so that the momentum of the atom is conserved before (it is $0$) and after the absorption ($\textbf{p}_\text{photon} + \textbf{p}_\text{recoil} = 0$). What am I missing here?