Question:
Answer:
In order for a sacrament to be valid two things are required: valid matter and intention.
The Church interprets valid intention widely. For valid intention, the person simply needs to intend what the Church intends.
The Catechism of the Catholic Church:
This is the meaning of the Church’s affirmation that the sacraments act ex opere operato (literally: “by the very fact of the action’s being performed”), i.e., by virtue of the saving work of Christ, accomplished once for all. . . . From the moment that a sacrament is celebrated in accordance with the intention of the Church, the power of Christ and his Spirit acts in and through it, independently of the personal holiness of the minister (1128, emphasis added).
As St. Thomas Aquinas notes in his Summa Theologica:
Consequently, others with better reason hold that the minister of a sacrament acts in the person of the whole Church, whose minister he is; while in the words uttered by him, the intention of the Church is expressed; and that this suffices for the validity of the sacrament (III, q. 64, a. 8).
In the situation you describe wherein the priest is baptizing but his mind is wandering, the sacrament would still be valid. The overall intention of the priest is to baptize as the Church intends, even if his mind wanders during the sacrament the intention still remains.