The phrase "a solemn expression" is correct and usable in written English.
It can be used to describe a facial expression that conveys seriousness, gravity, or deep thought.
Example: "Upon hearing the news, she wore a solemn expression that reflected her concern."
Alternatives: "a serious look" or "a grave demeanor."
He sat today at the defense table with a solemn expression that rarely changed.
Sitting among them, in a dark suit with a solemn expression on his face, was a seriously jet-lagged Mr. Clinton.
Father's face changed, a solemn expression suggesting that he might be taking offense, which of course made the whole thing funnier.
Eighth-Grade Reading (1998) 1. Acceptable (57 percent): "I imagine a girl wearing too much makeup and a boy with a solemn expression on his face as he tries to pay with an orange".
Pat O'Connor's "Fools of Fortune," which opens today at the Carnegie Hall Cinema, wears a solemn expression that is appropriate for a sad romantic melodrama set against the background of Ireland's fight for independence.
The portrait is a slight departure from the 2009 version, which featured a close-up shot of then President-elect Obama, with a solemn expression and a blander background.
The Museum of Modern Art featured it in a 1951 exhibition, 14 years after it went off the market, terming it "a most solemn expression of streamlining".
His fur was a dull silver, the color of dishwater, but he had this beet-red face, set in a serious, almost solemn expression.
(They are not shown here, but what is on view includes his lovely 1909 portrait of Pierre as a child, wearing a pink paper party hat that confers a joie de vivre belied by his solemn expression).
His solemn expression in the photograph is fair warning for his latest album, "When We Were the New Boys," which is a collection of cover versions.
